FYI - My 8 yr old daughter and I spent a pre-trip day in Heidelberg last year. We ended up going to the little zoo in town, which according to our ABD guides, not many folks do. It was a cute little zoo - definitely NOT touristy (therefore, not all signs were in English), but the cab ride there was so expensive, that I'm not sure I could recommend it (although I believe the dollar is a little stronger this summer). Might be nice for a family with younger kids, but think it would only keep you occupied for a couple of hours maximum, and be sure to check the cab fare to judge whether it's worth it! There are quite a few boat trips, and the walking along Philosopher's Way. We only had time for shopping/touring old town and a boat trip on our free afternoon the following day after the ABD tour of the castle. So you might want to go ahead and do one of the above. I think there are also walking tours of old town, which we didn't have as part of ABD, and you can rent bikes for the more adventurous! All of this is walkable from the hotel. Enjoy - I really liked Heidelberg!