I will second the please be careful on the sand comment. It's a good way to tweak your already tweaked knee.
I have a windsurfing/cycling friend that broke her knee very badly at the end of August. She was totally out of comission and no allowed to weight bear at all on the leg. She managed to do Pilates with her brace and we had her in a pool in water up to her low chest area doing exercises for her leg after about 3 weeks. She would walk forward and backward, side to side, do squats in the water, leg lifts in all directions, knee bends......You should be able to so something like that pretty quickly but clear it with the Doc first.
My friend also took up knitting which helped her deal with the injury a lot. And, she lost 10 pounds in 6 weeks even though she wasn't doing anything, so I wouldn't worry too much about blimping out. Like others have said, you metabolism will be up for a while to try to deal with the injury.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS