The Harley shop on the corner of Salaam St and Passport Rd hires bikes. They have a price list on the counter. Pretty reasonable prices too. The South African bloke who runs the joint, Rennie, is also a good guy and the best bike mechanic in Abu Dhabi. He is involved in the bike racing scene if that takes your fancy but there isn't much in the way of touring around AD. There are the sand dunes at Liwa but very little else. You might also pick up a cheap bike (600 sports bikes mostly) in the Hiltonia carpark on the corner of the road out to Marina Mall. The rich kids thrash their bikes, get bored with them, and occasionally sell them for a song from this carpark. But the Harley shop is the only real bike shop in AD (ie staffed by bike riders not clueless sales eggs), and a good place to start if you need wheels for a short time.