You really can cruise, have a great time and do lots of things without spending a lot of money after you pay for the cruise and get to the port (which can be costly if you fly in)Pre cruise or post cruise hotel and meals.
All of the shows are free. If you are on a Voyager or Freedom class then there are the ice shows. There's minuature golf, dancing, ping-pong, rock climbing wall and all sorts of poolside events that are free. Pictures can get costly if you go overboard on them. Also, you can keep tabs of your account on most newer ships(not sure about Explorer) by checking on your TV daily. That way it won't sneak up on you at the end. Cruising on a budget is just like being on one at home. It takes disipline and I know from experience that you can get caught up in the moment
Enjoy your cruise.