get a tourist visa if you're Canadian or American - you can stay for 90 days. To extend, daytrip to Colonia, Uruguay and renew on return.

can i buy my visa at the border if i'm American or Canadian, or do i need to get it ahead of time? - andy

as of aug 2009, you get your visa at the border (i.e. when you land in argentina). there's no fee involved either - just a simple piece of paper stapled in your passport that they remove when you leave. - kareem