Hikone - things to do?

Hi,
I'm considering visiting Hikone as a day trip from Kyoto.

How similar is the castle to Osaka Castle, or Nijo-jo in Kyoto (both of which we've already planned to see)?

A brief search on the web hasn't resulted in much "to do" whilst there. The castle/museum/garden and the shopping street.

Are there other things like onsen? Views of the lake or water activities (tours, as opposed to jet skiing)?

How similar is the castle to Osaka Castle, or Nijo-jo in Kyoto (both of which we've already planned to see)?

The three are all completely different. Osakajo is has a modern, concrete keep and some of the largest grounds, moats and walls of any castle in Japan. Nijo does not have any keep, at all, anymore. Instead it has the best preserved castle palace in Japan. Hikonejo, on the other hand, has an original, wooden keep, as well as interesting, wide castle grounds and a partially reconstructed palace building (which is nothing compared to Nijojo). The keep is much smaller than Osaka's, but the fact that it is an original one, made out of wood, makes it completely different from Osakajo and Nijojo, and worth a trip for a castle fan who has not seen an original castle keep before.

A brief search on the web hasn't resulted in much "to do" whilst there. The castle/museum/garden and the shopping street.

The castle and adjacent garden is the only major attraction of Hikone. But it is enough to justify a half day side trip if you are interested in castles.

Boat tours of the castle moat are offered, but I am not aware of any activities on nearby Lake Biwa.

The main tower of Osaka castle is a modern reconstraction, made of concrete.

Nijo castle is a palace-style castle. Main tower is lost.

The main tower of Hikone is original.

Well, just to add, if you like, you can ride a ferry to chikubushima Island.
http://www.gojapango.com/travel/shiga_chikubushima_island.htm