I have been on about 10 or 12 cruises over the years, including before they became as popular as they are today.
I will answer your inquiry about getting lucky first. I usually travel with the wife so that lets me out of that scene, although I know from talking to a few guys that they usually did very well, as the other posters have implied.
As for the tours, when I am in a port for the first time I usually book a half day tour, to get a feel for the port, then go out on my own after a quick lunch onboard. If I have been to the port before I usually just do my own thing. Remember some ports are a long way from any activity. Caracas for example is an hour taxi or bus ride from the port. Martinique is a short walk, about a half hour same for Cozumel. US Virgin Islands you are in town at port as you are in Ocho Rios. Check out the ports before you leave. Usually the itineries have a beach port, a shopping port and a historical port. If they double up on any it is either the shopping or beach ports usually.
Personally I like the 3 or 4 port cruises in a week. It leaves you time to relax between, and I love the days at sea, very relaxing. I have done a two week cruise in the Baltic and it was fantastic, lots of cruising, lots of interesting ports. Best trip ever.
As for the tab, beware. As the others have pointed out. If you are not used to runnign a tab you can get bruised easily. Personally I was used to the idea before cruising and I have never had a bar tab bigger than a couple hundred dollars, but then I don't drink alcohol and the wife is a cheap date. The only time I had anything bigger was on the two weeker and the only way to purchase things on board was with the shipboard account, that included the jewellery and souviners in the ship. I think that bill came in at about a grand.
As for the entertainment. LOTS and something for everyone. Carnival is a great inexpensive line to get a feel for cruising. The crowd is usually split in age groups, but with 3000 passengers on board most ships these days you will find it easy to associate with like minded people. I have found that the younger and older groups tend to be the busiest. The middle aged groups tend to be in bed early and up early, making the most in their minds of the dollars invested.
If you compare the dollars spent on a cruise to a resort all inclusive or otherwise, the only real difference is the booze. On the ship though you get to see different places in the same time frame.
Personally I look at cruises as the only way to travel. All you have to do is eat sleep and breath. They do everything for you.
Have fun and try to experience as much as you can. Remember you can always sleep when you get back to work
