We stayed for one week late august at the Hotel Catullo. Very centrally located with garden onto the lake providing a quiet escape from the crowds during the day. Numerous little alley ways, most of which lead to the lake with lots of little shops, restaurants etc. The castle is worth a look, with free admission for over 65yrs, as are the Roman Ruins with admission the same. Take a boat trip around the peninsular or a ferry to one of the other ports, especially on a market day for a bargain.
We have holidayed twice before in Sirmione and have just booked another two weeks next augusst.