I don't know much, but I'm gonna add my worth anyways

I'd say that a decent specced Mountain Bike would be a good suggestion - I do many miles on my Land Rover and (with the right saddle) it's incredibly comfortable. I don't know much about cruisers, but I would say that from this thread they sound fairly similar to the MTBs. I spent about £350 on buying it, and its done 1200 miles faultlessly, aside from a couple of failed bearings and a stretched chain, although that's probably due to me riding it like a bit of a loony

And before people say they're slow, I keep up with and overtake people on it anyway!