Kyoto? Two Words. Johny Hillwalker. That's a good half day and one of the best walking tours I've done anywhere. It's full day if you continue on at the end of his tour and do the Philosophers walk on your own.

I'm in Osaka as well most of next week (at the Sheraton Miyako no less) and have been deliberating side trips. I've already done Kyoto, so I'm looking at side trips like Mount Koya, Nara, Himeji, etc.