There are hand bikes but it is hard to get your heart rate up without any leg use.
Maybe try some wheelchair racing.
About the knee, my right knee had a meniscus tear, so during surgery they took out much of the cartilage on the inside of the knee so they recommended doing things that are very low impact because my knee is already going to wear out faster than a normal knee, so don't wear it out any faster than it needs to be.
Does your gym have a handbike? I find them really good.
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OT; there are some fairly decent exercises you can do at home that'll get your heart pumping if you do close sets. Things like squats/push up combinations ect. Not the most entertaining form of exercise though.
Swimming is the best idea advanced so far. Some kind of low-impact routine with light wrist weights might also be good, you don't bend your knees too much.
Swimming is probably your best bet, cyclnig is good, but steep hills will probably hurt your knees, if its cold out just wear some good layers, with a wind/water proof coat and after about 4-6 miles you'll warm up enough.
if its just general fitness you're after trying just doing bodyweight exercises, something like fitness666.com could well help, there's no impact, and the only ones you might struggle with knee wise are squats, but if you start off low like the site says you'll soon build up extra muscle around the knee to help support the joint