in addition to biking (usually every day as that's my main mode of transport) i've been doing a sort of HITT workout that i can really vouch for. no weights are required, just a bit of space, and you'll need a deck of cards. assign an exercise to each suit (or color, if you prefer) -- ideally these exercises should incorporate multiple muscle groups but you can try whatever you like. then flip a card and do that number of that suit's exercise. jacks, queens and kings are 11, 12 and 13 respectively. aces are cardio -- i do jumping jacks or some jump roping -- and the next card is doubled. do this with as little rest as possible between each card. it's a great workout that really gets my muscles burning. i've only been doing them for a couple of weeks, 3 or 4 times a week, but i'm already starting to see some muscle growth/fat loss.