Try Joe's bike Rentals on Middle Street(near the airport). I've rented there many times, usually $5.00US a day. Weekly rates a little less. Not sure if he's open Sundays though.
I like to find a nice bike someone is renting, then ask them where they rented it. There is a wide range of rentals from junker bikes to brand new bikes with fleece padding!! The nicest bikes might not be the closest. Belize it!
Joe's bike rental by a mile. They will charge you about 35-40US per bike for the whole week. And they have just renewed their entire fleet. Call them on 2471 once you arrive.
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