The 100 push up challenge:
The 200 sit-up challenge:
The 200 squat challenge:
All are kind of fun to do on top of your regular stuff.
Also look at getting into rock climbing. I find it was helped me tone up quiet a bit. Plus it is a lot more fun then just going and lifting weights.
And for core strength, this may sound lame, but sit on a burst resistant stability ball instead of a chair when you are at your desk. It also helps to improve your posture. Plus you can bounce to music :)