Any suggestions on the most interesting sites to see in the Texas Hill Country area? Besides Austin, that is! Planning 4 or 5 days there at the end of March.

Well, you could try Fredericksburg, did you see this on Wikitravel yet?

http://wikitravel.org/en/Fredericksburg_(Texas)

It's a very German area, so I would check out the brewery...and I'd TOTALLY go to the "George Bush Gallery"

Also, here are some things outside of Austin to check out:
http://wikitravel.org/en/Austin#Get_out

Have lived in Fredericksburg and we still own a house there. Since we spent most of our time fighting the city concerning animal issues (they had the animal pound at the town dump for convenience) I am not fond of the town. However, it makes a nice day trip and the beer is quite good.

Today I am sitting in Georgetown, north of Austin, well know for the first successful prosecution of a KKK member in TX. The courthouse for that trial is on the town square and the wood alone makes it worth seeing.

I prefer the smaller towns, Gruene (oldest dance hall and used for a scene in the movie "Michael"), Comfort is an antique town and no one should come to the area without making the drive to see Luckenbach and down a beer.

Oops - forgot to add this, smalltowngems.com has many pictures of Fredericksburg downtown, Comfort, Boerne, Gruene, Georgetown and a few others in the hill country. That might help with your planning.