Wheelchair Help ASAP!?

I am going to have some surgery this Wed. and I will have to be in a wheelchair for about 3 weeks.I'm not allowed to bear weight on my leg for about 3 weeks.But, I don't want to gain any weight while I'm in the wheelchair.
I exercise daily, but with the chair I won't be allowed to.
I'm about 125lbs.5'7". 20yrs old.
But recently I've only been eating about 400 calories or so .
Can anyone give me advice on how to not gain weight?

Please Help ASAP.
Thanks.
^_^

Yes, I can do upper body exercises.
I lift weights on a daily basis (7 days a week).

Cardio stuff.

Swimming, aquarobics

Arm exercisers (pedals for the arm) or those funky little bike-like pedalers for the legs/leg

Theraband exercises- try Bungies, etc. if you want.

Moving your chair burns a lot, and you can do chair push-ups (push down with your arms and try to lift your bottom out of the chair), etc.

Can you do upper body exercises? Like passing a medicine ball? Or lifting weights?

Well for starters you metabolism has already slowed down a ton, your should consume no less than 1200 calories a day to keep your body running normal and healthy. Anything less and your body will start storing fat, eating muscle, and your metabolism will slow down a lot. Try eating 5-6 small meals everyday, to give your metabolism a boost. Make sure they are healthy, lots of fruits and veggies, meats and dairy. You can do pull-ups for exercise, and ask about being able to do cycling, you wouldn't have to put any weight on your leg for that? Make sure you stretch a few times a day to keep your circulation up, and your muscles in good shape, lift free-weights... These are the only things I can think of, ask your doctor before the surgery, they should know of a few things you should be able to do.
Good luck
Toni Lynne :)