Hi Amy,
I've spent a lot of time in the Lombardia region while visiting friends. They have taken me to some interesting places:
*The the Museo Casa del Podestà in Lonato del Garda (there also is a cool castle in this town but it is a lot of walking and not sure how your knee is right now). Here's a link to my
blog post for Lonato del Garda (scroll down past my blabber at the top of this post!).
*Il Vittoriale degli Italiani in Gardone Riviera (although again this could be more walking than you are up for). It really is an interesting villa though.
*Museo di Santa Giulia in Brescia (small museum but very cool - Roman ruins from 2 Roman Villas).
*Castell'Arquato - fun little town wtih a very cool castle - not too much walking if I am recalling correctly for this castle. Beautiful views from the top!
Some other more well known places I would also recommend - Bergamo Città Alta
and Cremona.
For a lake idea, I would recommend Lake Iseo. Much less touristy than Lake Garda. I wrote a
short trip report about my stay in Iseo as well as other visits to the lake. I also wrote a bit about a visit to an abbey and a monastery located along the Cluniac road of Franciacorta in this trip report. I am not a wine drinker (migraines) but the Franciacorta region is a very famous region for wines in Italy.
Lucky you! I've always enjoyed my time in this region of Italy!
*Edited to add... not sure what the boat schedule is for May on Lake Iseo.