... We have been invited to cruises with friends, but these are "veterans" who want to do 2 week cruises, many days at sea, etc. Something we do not want to try the first time out.
At the moment we are considering river cruises (like on the Mississippi) a short 5 day cruise out of Baltimore (we can drive to the port and no added airfare), or a 3-6 day cruise out of Fort Lauderdale (add it onto the next time we visit family there)...
So the thing with short 3- or 4-day cruises out of Fort Lauderdale is that they're not like other cruises. They tend to attract people who are looking for low cost trips with easy access to lots and lots of booze, and it can be a bit like being stuck in a non-stop spring break. If that's the type of vacation you enjoy, no problem and I am not judging; but I can tell you I have several family members who are certain that cruising is not for them after doing one of those trips.
I think the short trip out of Baltimore is less likely to be party central; or maybe a one-way trip to Quebec would give you a good taste of cruising. A week from Vancouver to Anchorage is also great. If you want something more tropical and are willing to fly, then a week-long Caribbean cruise out of San Juan will usually only have one sea day, and Windstar cruises out of St Maarten have no sea days.