I agree with the above poster who mentioned taking 4 or 5 day cruises out of Galveston or New Orleans. I live in N.O. and have done the shorter and longer cruises from here and have always had a good time. The Carnival boat from here leaves on its 4 day cruise on a Monday at 4:00 p.m. and is back in port the following Thursday (and your off the boat usually by 10 a.m). I believe you have one or two stops in Mexico. The 5 day cruise leaves the port on a Thursday at 4:00 p.m. and returns to the following Monday (off by 10 a.m.). I am sure that the Galveston boats pretty much follow the same schedule.
I have used a website called 1-800-CruisesOnly.com religiously for years and have never had a problem with them (at least for the last 7 cruises I have bought from them). I also like their website as you can do an advanced search using many differnt parameters. My favorite is to pick the port I want to leave from and look at what my options are. 4 & 5 day cruises aren't usually expensive unless.
I do have to agree with another poster above, if you can afford it and take at least a week off, I would also tell you to cruise Alaska. It is a completely different place and very beautiful. If your going in July/August, you get sun 24 hours a day for 2 months straight. It is something to experience.
Now if you can't afford Alaska, then a 4 or 5 day cruise to the Western Carribbean will certainly let you know if you love or hate to cruise. Good luck and have fun.