I'm guessing Tigre is still most conveniently accessed by the picturesque rail ride - no need for cars and the like, particularly since I think 50% of Porteņo males seem to be channeling the spirit of Juan Manuel Fangio, but without the driving skills.

(Who's he? You gotta have a certain, er, maturity to know.)

Colonia del Sacramento can be a nice day - crowded on weekends, dead on Mondays for sure, and of course take your passport and be prepared for international formalities. Take the fast Buquebus boat. Only. And be sure to check the promotional rates on the website.