i vary my exercises all the time. i will go jogging one day, do intervals the next, then go biking the next day and so on. i try not to go to the gym more than once a week now. i got really bored with going there and lost my motivation to workout for a while, which really set me back. now i enjoy the diversity, which makes me more motivated and i'm seeing better results in my fight against the fat.
i've been following the 100 Push Ups challenger in the forum
100 Push Up Challenge . i might try the challenge once i lose a little more weight - it sounds like a great idea.