I believe the Getaway Package (bike, snorkel, floats) was around $30-, $6.50 (?) just for the bike rental and I believe this can be booked in advance.
As far as the time limit on the bikes go, I wouldn't worry about that. They have plenty of bikes (all sizes) there. It goes something like this: There's a couple of CM's at the bike rental booth that take your ticket(s), then they show you a bike trail map, then you just walk over to where the bikes are, pick what you want and just go! (no sign-out of bikes or start time recorded; pretty laid back process)
BTW, they don't hold you to your reservation time on your bike rental ticket, We went around 2hrs after our 'assigned time' and was never questioned.