My favorite day trips from Madrid are Toledo (my clear #1), Cuenca and Segovia. Cuenca is practical only via AVE, which would be costly with a last-minute ticket purchase.
I haven't been to El Escorial. To me Avila is a very distant 4th unless you want to include Salamanca in the mix, in which case Avila is #5. Avila has a wall to be walked on, which is fun, but to me it doesn't need much more than 2 hours to see. The others are full-day (or longer) destinations.
Many of the modernista sites in Barcelona are subject to very long ticket lines, so you'll benefit greatly by buying tickets in advance. You don't generally need to buy those tickets before you leave home, just before you plan to visit the sites. Most of the problematic tickets can be purchased from the tourist office under the Placa Catalunya at a service fee of 1 euro per ticket. These are the sites I'd be concerned about:
La Sagrada Familia. Separate ticket required for tower, if desired; try for first entry time for church itself. It gets crowded fast. T.O. only sells church tickets, not tower tickets.
Parc Guell. Consider weather. If it's hot, early entry is good. If it's cool you might prefer afternoon. If you want to see the little gatekeeper's cottage, you'll want to buy first entry time and go there immediately to avoid a line.
Palau de la Musica Catalana. Requires a tour. English is available. T.O. doesn't sell Palau tickets. Not Gaudi but I'd consider it a near must-see (as is the already mentioned and blissfully uncrowded Sant Pau).
Casa Mila/La Pedrera. There's a lot of info about other Gaudi sites on the top floor. If you're interested in all that, you will need a lot of time here, perhaps 3 hours or so.
Casa Batllo. Always packed.
Picasso Museum. Not a modernista site, but advance tickets are a must. (Alternatively, the T.O.'s Picasso Tour gets you in.) The most crowded art museum I've ever seen, though I haven't been to the Vatican Museums lately. If you don't care about Picasso, by all means skip it.
Palau Guell. Generally not a terrible line, but there was one the first time I walked past. I'm not sure whether the T.O. handles these tickets.
The tourist office under Placa Catalunya runs modernisme walking tours. They aren't particularly detailed, but they're inexpensive (I think 16 euros). They don't go inside any of the modernista sites, but they do give you a free 10-15 minutes in the small Modernisme Museum just before it closes. I'd have liked more time, but it was still a worthwhile visit, and I didn't feel I needed to return later and pay for admission.
The excellent and huge Museum Nacional d'Art de Catalunya has (among many other things) a modernisme collection that you would enjoy. While there, take time to visit at least the church frescoes.
Do not spend money or time going inside the Casa de les Punxes.
Casa Amatller has a modernista exterior but a mostly pseudo-castle interior. It is interesting but will chew up a fair amount of time and is costly, so you'll need to decide whether you'd rather do that or spend the time wandering around looking at bunch of other exteriors. It's in the same block as Casa Batllo.
If you buy the Ruta del Modernisme book and map, you'll get a discount card that will save a bit of money on tickets to most of the modernista sites, but I think it only works if you buy the tickets on site, which is not terribly practical for the most popular places with long lines.
Assuming you're interested in modernista architecture in general, not just Gaudi, you'll need to do some prioritizing. I spent ten days in Barcelona, nine of them mostly devoted to tracking down modernista architecture, and I didn't have time for everything. Sure was fun, though.
If you care about food, do some advance research. There's more mediocre food in Spain than in Italy, for sure. Just walking down a non-touristy street isn't enough.
