Yeah it's a really good idea to switch it up a lot. The best way to overall looking better is to keep changing what you do so your whole body gets a workout you end up making more muscle and the more muscle you have the more base calories you burn a day. It looks a lot better in the end then say, just lifting weights, having buff biceps and a flabby stomach heh.
If you're really out of shape try and just run for like a half a mile then the next day walk a mile or two. My body knows when i'm about to do some exercise and gets in a mode, or at least thats what it feels like, and at first your body might be going wtf are you doing?!
Oh yeah and one more thing don't do strait one thing like 50 sit ups, do like 20 sit ups, 20 push ups and repeat 2-3 times