There are several places that rent bicycles. The most reasonable one we found is Joe's Bike Shop, a great place to rent a bike. He is just down the street north from Ruby's and across the street from the Pier Lounge. He offers his bikes at $12 Belize dollars for 3 hours which is only $6 us dollars and the longer you have the it gets even cheaper, $39 for the week, day rates are $9. Take a walk towards Ruby's Cafe and if you ask anyone they'll point just down the street to Joe's bike shop.
Please note that bicycles must have an attached bright white light to be ridden at night.