It's hard to say without knowing what parts of your body the surgery has affected. I've seen little machines that are like a box with pedals sticking out that you can use in lieu of a stationary bike while the rest of your body relaxes on a couch or chair. However, the least expensive one of those I've seen was $80. If you're unable to work your legs, maybe you could use some light hand weights and work your arms a bit to burn at least some calories.
Maybe your doctor can suggest some low-impact exercises that won't cause any harm. Other than that, watching what you eat will be the most important thing--finding that balance between nourishing your body enough to heal, but not indulging excessively. You don't seem very far from your goal, so I would think it's possible (without knowing the specifics of your situation of course). I wish you a speedy recovery.