I went on my first cruise (as an adult) recently, and couldn't believe how much the whole thing was designed to keep selling you things -- art auctions, casino specials, massages, fancy dinners, bingo, etc. Every day there was some sort of free activity or raffle that was actually a trick to sell you more things! We did some excursions and bought a souvenir photo and a couple of drinks (in addition to the bottle of wine per person you're allowed to bring on board -- check your cruise line!) It wasn't bad, but I think we decided that cruising's not really for us anyway. Regarding excursions, I just went on tripadvisor to look for highly rated things we wanted to do and then booked on their websites. There was really not much to do in the ports we went to, so I'm glad we booked excursions.
Booking tip: Once we knew what cruise we wanted to take, we went to cruisecompete.com and put in our info. They farm it out to a bunch of travel agencies who all write you back with quotes. One of the agents quoted us the same price as the cruise line (and expedia, priceline, etc.), but with $125 of onboard credit. Score!