One thing's for sure, when it comes to Madrid there are a couple "top tier" day trips (Toledo & Segovia) that are really fantastic and then a few others that are pretty great (Ávila, El Escorial, Cuenca, maybe Salamanca although I've heard it deserves more than a day). Last year we explored all of the heavy-hitters, so this year we've been reaching a bit further, which means we've mostly been going to smaller towns that are a little more off the beaten path. Once we've exhausted those that can be reached by public transit, we may have to repeat places we visited last year or go to places that we consider just a bit too far for a day trip (2.5+ hours plus one way) and stay for one night.
All that being said, this year we still have plenty to see. So far this year we've been to Aranjuez, Cercedilla, and now, Chinchón. Chinchón is 50km southeast of the city of Madrid and has a population of about 5,500...so it's pretty small. We went one sunny Sunday after church with a small lunch in tow, and figured we'd stay for the rest of the day, but truth be told, we hardly stayed an hour and a half. There's just not a whole lot to see or do, but! But! Don't cross it off your list yet if you're calling Madrid home for a while. We were still glad we took the time to check it out because it's a rather peaceful and picturesque place - it just doesn't take long to see the whole place.
While we were in Granada, we were talking with our host about her dream to open up a hotel at some point, and she mentioned that her friend wants to open one in Chinchón. We told her that we'd recently been there, and that we liked it a lot, but weren't sure that we would want to spend more than half a day there. Even if Chinchón doesn't boast the cathedrals and castles of Toledo or Segovia, it's still a pretty special spot with stone streets, a central bull ring (that was used just the day before we were there!), and classic Spanish architecture.