Metronome is great to learn how to play on beat but drum loops are the next best thing to playing with a band.
I'm sure if you google drum loops you can find some free ones on the net.
Just put on a drum loops and try to play a chord on each snare hit, then every bass drum hit. Then play every other hit. Have fun with the loops and you'll actually end up working up a pretty good exercise.
When I want to write some new riffs I do that exact same exercise. I start with the good old E5 and add others as i go.
Hopefully that gives you one more exercise to work on rhythm.
