Luss is always good for a wee trip, it's better in the summer when you can actually use the beach but it's a nice place for a wander around - very twee and picturesque. Mind you it always seems to rain when I go there....
There are some nice places up the east side of the loch, Drymen, Balmaha and the like. They're a bit less touristy than the south/west sides.