When we got to St. Croix, we considered renting bikes until we got to the end of the pier where someone was handing out flyers for renting scooters (mopeds). I was very leary at first because I had rented quads before and felt it was really dangerous.
St. Croix is really safe for mopeds. The drivers were so courteous, I couldn't believe it. Renting bicycles would have been more dangerous because you do have to share the road with motor vehicles and they were constantly passing and would be hard to get up the hills into the rainforest.
The cost is $55 for the day (until 4pm) if you want your own scooter or $75 for the day if you are willing to share with someone (helmet and map included). That's a whole day at $37.50 per person. The pedal through paradise (on bicycles) offered by Carnival was $51/person. So I thought this was a bargain. You can go all the places the bikes go and much more. We saw Sprat Greathouse, botanical gardens, rainforest via Mahogany road. We found the bar where you feed the pigs beer (non-alcololic of course), but it was really funny. There was plenty of time for other tours, but we weren't sure where we were going, so we just cruised around near Fredericksted.
I highly recommend it, it was the best excursion I've ever done.