Hello peter 1,
yes I'm back and in catching up, just came across your thread.
I think SailorJack's advice was excellent, as usual; a ten day is a great canal cruise--but go all the way through the canal. To me there is something anticlimactic about transiting up to Lake Gatun, then turning around and going back; although this will definitely give you an idea of how the canal works.
The first canal trip I took was in 2000; it was actually back-to-back canal and Mexican Riviera cruises (18 days) on Sea Princess. The canal trip was Ft. Lauderdale to Acapulco. But since I live near Los Angeles I figured it was easier to stay on and cruise home rather than go through the hassle of flying from Acapulco; and besides, I like longer cruises and lots of sea days. I liked the trip so much I'm doing it again next April, but only 15 days this time.
You don't mention where you live, but that, along with how long a cruise you want, would be big factors in your choice.
If you live near Florida or Southern California I would encourage you to do as I do, fly to the cruise, then sail home.
If you don't live near a port, then you really have more options since you will have to fly both ways anyway.
But I would encourage you to do the full canal transit, and preferably on a ship--swimming it dosen't sound like much of a vacation.