Salamanca is rather far from Madrid for a day-trip, especially considering the size of its historic district. However, there is a 96-minute train option if you are prepared to get yourselves to Chamartin Station early enough for an 8:55 AM departure.
There is truly not much to Avila; I would skip it since your time is limited and you have a lot of potential day-trips. Toledo is the most interesting and is worth an overnight. You could also add Cuenca and Alcala de Henares to your list. Cuenca, however, is viable only if you take the AVE train, and that will be too expensive for a day-trip unless you're able to buy a cheap (non-refundable) ticket way in advance.
Zaragoza is often blistering hot in the summer, but I'm guessing you didn't find a $317 fare in the summer. If this is a winter trip, I'm not sure what Montserrat and Cadaques will be like (though they might be fine). Four nights in Barcelona is pretty minimal. You may or may not decide you have time for day-trips there. Many of the most popular sights pretty much require that you buy tickets ahead of time (to avoid very, very long lines), and it's hard to cram a lot of sights into a few days when you're having to guess how much space to leave between them. The problem sights I'm aware of are: La Sagrada Familia, Parc Guell, Casa Batllo, Casa Mila/La Pedrera, Palau de la Musica Catalana and the Picasso Museum.