Kim, here's what ya need to do:
Prepay all your tips. If you drink wine prebuy enough for the cruise. If you like mixed drinks, Carnival will sell you drink coupons. Prebuy all your shore excursions. If you drink soda, buy a soda card. Get a spa menu and if there is a service you want to take advantage of it, then pay in for on board credit to cover it. Voila! no hidden fees. Done!
My advce for GBOS is to go with the flow. Don't over analyze, and let yourself just have some fun and everything else will work out. I would bring $100 day as a basic rule, but it also depends if you drink alot, do the spa stuff, like to donate to the casino etc... You'll be charged $10 daily for tips usually.
If you're lucky enough to have someone special say "yes" then just remember, it really is like riding a bike, you won't forget what to do. lol!