I'm in Buenos Aires for a couple of days and I'd like to go to Montevideo for a day trip. I've seen that there are ferries and planes between the two cities, but are there also other options? And which of these options is normally the cheapest and which the fastest?
There are regular and express ferries from Buenos Aires to Montevideo.
One option goes directly to the Ciudad Vieja district of Montevideo, situated very close to downtown, with the Buquebus ferry company, and takes about three hours.
Another option is the ferry+bus combination (which I've personally done in the reverse direction), which has two companies - Buquebus and ColoniaExpress, which takes you to Colonia, and then down by bus. It's cheaper, but takes an extra 1-2 hours depending on crossing times and connections.
There's actually really good information and links on Wikitravel's Montevideo article.