I'd suggest Okayama as an alternative to Osaka. There are several rewarding day trips from there, including the aforementioned Kurashiki. The local train from Okayama takes you straight to the historic area in about 15 minutes.

Both the famous Korakuen garden and Okayama Castle are right on the river so might be a bit cooler.

From Okayama, one could even do a day trip over the bridge to Shikoku on the Marine-liner.