Places for which you want to have tickets in hand when you arrive at the site because of dreadful ticket lines and possible sell-outs:
Barcelona -- La Sagrada Familia, Parc Guell (no tickets sold at site now), Casa Mila/La Pedrera, Casa Batllo, Picasso Museum, Palau de la Musica Catalana (English tours may sell out). Unless you pay extra, those will all be timed tickets. All but the last get extremely crowded. Recommend booking one of them (if interested) for the first time-slot each day. Parc Guell could be quite hot at mid-day. As far as I know, none of those places sell out really early, but the early-morning time periods are popular, so those tickets usually disappear first. There's a soccer-related thing called the "Camp Nou Experience". I don't know anything about it, but it sells out extremely early. One highly-recommended site you can still just walk right up to is the Sant Pau Modernista Site. It's a multi-building complex (former hospital) not far from La Sagrada Familia. Amazingly uncrowded for Barcelona. Allow at least two hours for an un-rushed visit. There are a couple of especially beautiful rooms at the end of the self-guided (audio) tour; they shouldn't be missed.
Seville -- Alcazar and Cathedral. There is a separate tour of the private apartments at the Alcazar. It doesn't get rave reviews, so I didn't bother. If interested, that ticket will need to be bought very early; capacity is limited and it does sell out way in advance. The regular Alcazar tickets don't sell out really early, but the line looked hours long last April. The Cathedral situation can be managed by purchasing a combo ticket at the Iglesia Colegial del Salvador: Buy ticket, see church, walk to Cathedral (about 1/4 mile), show ticket to line-minder and walk right in.
Granada -- Alhambra. Tickets sometimes sell out months in advance. September tickets may go on sale in May or June; I'm not sure. Monitor
https://tickets.alhambra-patronato.es/en/ for ticket availability.
Madrid -- I didn't observe ticket-line issues at the Prado in May 2015, but things can change, and occasional travelers do mention encountering a bit of a line. I don't know whether advance tickets are time-specific or just date-specific. There's a combo ticket covering the Prado, the Reina Sofia and the Thyssen if you are art junkies. It doesn't save a tremendous amount of money. There are never lines at the other two museums, just a clot of people standing in front of "Guernica" at the RS.
Toledo -- I ran into an annoying ticket line at the Cathedral in 2016, but I hit Toledo on a religious-holiday weekend in May. No one else has reported a significant delay there.
With reference to tours: Viator is a re-seller, not a tour operator. It reportedly charges tour organizers around 25% commission. A lot of travelers use Viator for research, then Google key phrases from the tour description to try to locate the website of the company running the tour. I have heard that sometimes means a lower cost but don't know for sure; I don't take many tours. I did very much like the Barri Gotic tour run by the Barcelona Tourist Office. They limit the number of tour members, whereas the "free" tours take all comers and can end up with huge groups.
I think tours are problematic at all the places for which I've mentioned ticketing issues, because those spots are totally mobbed these days. (The situation in Barcelona deteriorated massively between 2016 and 2019.) You may need extra time to see what interests you; just getting close enough to the wall to read the labels in the Picasso Museum can be very difficult. Anyone wanting to make photos not full of other tourists will be driven wild by the need to keep pace with a tour leader.
The Alhambra is the most problematic place to tour with a group, in my view. You are allowed only one entry into some specific areas, so if you feel the group rushed through something you want to revisit (such as the interesting museum in the Carlos V Palace), you may not be able to return to it after the tour ends. The Nasrid Palaces are the most popular and crowded area; they are what everyone wants to photograph, and that doesn't work well on a tour, because you simply must keep moving along. If the Palaces are not the last stop on the tour (which would allow you to remain inside, though you'd be swimming upstream to go back to the beginning), you will not be able to spend extra time in them. Another issue at the Alhambra is that I think the typical tour is about 3 hours. There may be a toilet break along the way, which would mean even less tour time. The complex is huge; one can spend nearly a full day there. Finally, there's the fact that various parts of the complex are open at different times of the day; you might miss something you want to see because it's not on the tour but is only open during the time of the tour. If you visit on your own, stop at the information desk to find out the hours of the various buildings.
Audio guides are now available at, seemingly, every site of tourist interest across Spain. I've found them generally to be good, and I like the flexibility to move at my own pace. I didn't use the audio guide at the Mezquita in Cordoba, because Rick Steves himself admits it's not a good one. I was perfectly happy to depend on the audio guides at both the Alhambra and the Seville Alcazar.
I haven't been to Montserrat yet, but Barcelona natives insist it's easily done on your own, and no one has ever reported having problems doing so after following that advice. You just go to the train station beneath the Placa d'Espanya and buy a combination ticket. The one that doesn't include the overpriced lunch is recommended. I believe you have two choices of transportation for the final leg up to the top (probably cog-wheel train and something aerial). I don't know anything about that except that you have to use the same transport in both directions. This sounds simple and avoids being cooped up on a tour bus. In do-it-yourself mode you'd have an opportunity to consider weather conditions.
I believe a lot of people take a bus tour to Toledo. I wouldn't do that. The train gets there fast. The central zone of the historic center (Plaza Zocodover to the Cathedral) is overrun during day-tripping hours. I have to assume that's where the tours spend a lot of their time. It's a beautiful city, but you need the freedom to walk always from the mobs and just wander around and visit some of the less-trafficked places. (In this respect it's a lot like Venice.) Note that historic Toledo is very hilly, which tends to slow ones progress.
Since you like tours, I'll mention the bracelet deal in Toledo. There are six or seven secondary sites covered by a single payment, for which you receive a plasticized entry bracelet. You can use the bracelet for more than one day as long as it stays on your arm. Mine survived at least one shower. At most--maybe all--of the covered sites there are short bi-lingual tours offered a couple of times a day. There were two beautiful former synagogues on the list. I thoroughly enjoyed those little tours. But you really must have an electronic map on a cellphone or tablet to find your way expeditiously from one site to the next when the tours are back-to-back.
I'd choose Bilbao rather than Pamplona without a doubt. Bilbao = more sights; Pamplona = more tourists. But I will admit the life-size statue of the running of the bulls is really neat. Hondarribia, east of San Sebastian, is an attractive side-trip. The fishermen's district along the water has the restaurants. The older (medieval) area is uphill. Bus service is good to both Bilbao and Hondarribia; I know nothing of the parking situation in those places or in San Sebastian.
