If you're starting to get bored with the same old routine then you may want to supplement your workout with a bit of jumping (considering your knees are in good shape). This is because any form of jumping (jumping jacks, jumping rope, plyometrics) burns major calories since you're pushing your body weight against gravity. I'm not saying that you should always do an entire workout of jumping because that would be exhausting, but rather add a few to your existent cardio program. This means that if you run regularly, then add jumping rope every once in a while. Or run for a few minutes and then do a minute of jumping jacks (or even better, jump squats) and then run for a few minutes again and repeat. The jumping is a good shock to your system, both physically and mentally, which keeps things interesting. Try it out and have fun.