Hi -- my MIL and her friend (in their late 60s and mid 70s and very active and able) want to rent bikes on Castaway. The friend once rented a bike with 21 gears on a vacation and she was nervous about riding it. Are these bikes like that or are they the kind with few or no gears? The terrain is very flat, right? I want to reassure them that they'll be able to ride on Castaway and have fun . Thanks for any information!