Didn't you ever see Royal Caribbean's advertisement - "Get Out There" ?!
It is very important not to generalize cruises - each cruiseline and each ship within the cruiseline, has unique characteristics. If you choose to sail HAL (Holland America), then you may feel there is very little activities after dinner - very sedated cruise. However, if you cruise a Carnival ship or a RCCL ship, you can party till dawn. Princess and Celebrity are somewhere in between.
I cannot emphasize any more -
DONT thing cruise has nothing to do - you have more things to do than you have the time. Most first time cruisers changed their view points after their first cruise.
DONT choose a WRONG ship - the ship's characteristics do not match with your personality - that would be a disaster in the making.
DONT choose an old ship unless for the itinerary. Newer ships (less than 5 years old) have far far more amenities and facilities than older ships. The price differentials are not big (less than $20-30 per day per person on comparable cabin category.
If you want to try a cruise, I strongly suggest you to pick your ship carefully.
There are so much to see at each port, and the modern ships themselves are like floating resorts.
Now I prefer 14days cruise than 7day cruise as the 14day length gives us enough time to slow down. The 7day is actually too RUSH. Though we are going on a 7 day on Dec 10.
In 2004 we did 4 cruises for a total of 38 days - one 14 day full Panama Canal cruising from Ft.Lauderdale to San Diego. one 7 day Mexican Riviera - R/T LA to Los Cabos, PV and Mazatlan. one 7 days Alaska SB (this by far the least favorable cruise because of the weather, though there were still lots to do at each port and there were 4 or may be 5 ports, cannot remember), and finally a 10 day X'mas cruise to South Caribbean.