Some people think if they exercise they should be able to eat anything at any time. If you're eating healthfully, it could be that you need to exercise more often. One of my sisters found that she needed to work out five times a week to see results. Everybody is different. I'm a college student with weak knees.
Knees are probably the most delicate and necessary joint in athletic activity.
I like to do exercises that mimic those naturally occurring movements you do in real life, such as squats, lunges and step-ups. You can't really strengthen the knee, but you can strengthen all the muscles around it. It not only gives me a good cardio workout, it helps me stretch and tone. Many hotels have VCRs. One or two water bottles.