I'm still an competetive rower. I can tell you that outside of cross country skiing (which is a great form of cross training in the winter) there is nothing like rowing.
If you live near a rowing club
http://www.row2k.com/links/links.cfm?cat=3) learn to row. Most club dues are less than 300 dollars per year, which gives you complete access to club equipment and the club house. If you are a member of an established club, and are visiting a city with a club, there are usually courtesies extended where you can use that club's ergometers, or even better you may get invited for a row.
The adult members of our club are rough split equally between those that rowed in college or on national teams and those that started rowing in their 30 's 40's and 50's. As far as preconcieved notions about elitism, I don't know of one club in this country that could be called a snobatorium. (but rowing does attract more than a fair share of eccentrics. I probably met more crazies on the rowing circuit than I did in art school.)
The concept 2 (usually called an erg) is a great machine and you can get a full workout in under 45 minutes. It does get kind of boring, and is almost impossible to watch TV (thank God for the ipod). Most rowers have love-hate relationships with the concept2. Its not in uncommon to her a rower say "ergs don't float".
Roller blades are also good and very portable. Swimming is fine, but if you are jumping in the pool for the first time, and are even in reasonably good shape, don't expect to be able to do a lot of laps. You will work out muscles yu never knew you had.