Segovia and Toledo are both easier day trips. I think Cordoba in a day would be a long, tough day. I was in Cordoba a few years back and the Mezquita is definitely something worth seeing, but I would almost think that Cordoba would be better as part of an amble through the south, rather than a day trip. For me, Toledo was a great starting point as an introduction to Spain when I first visited (age 14). Besides the knives, which were totally awesome, I think that it really provides a good cross-section of the history and culture of the country. Segovia is probably more visually memorable with the castle and the aquaduct. I re-visited Avila for the first time in many years recently and I had forgotten what a great place this was. Really can't go wrong on any of these.