Originally Posted by
greenythebeast
Would visiting Sevilla (2.5 hours by train each way) or Granada (4.5 hours by train each way) be worth the trip as a day trip?
Not really - both places are gems, and to do them justice you'd need a few days, I would think. Particularly Granada.
If you're based in Madrid for an extended period, Toledo is pretty much a must-do - it's a gorgeous old city filled with wonderful art. Sadly it's also rather filled with tourists, but them's the breaks.
Avila is a nice place to spend a more leisurely day. The old city walls are gorgeous.
Segovia, as has been mentioned, has some great stuff as well.
I guess if you've got five days for MAD and environs, you could do far worse than a couple of days in the city, a day trip to Toledo, and another day trip to Avila or Segovia.
For the day trips, you could also easily be back in MAD in time for dinner/nightlife as well, if you wanted to be.
Spain is one of those countries that you could spend a lifetime in and never see everything. I'd personally suggest picking a smaller number of places, really exploring them, and then going to other places on future trips.