All the hotels Avenida de Mayo are clean and cheap, the location is great. I stayed at the Ritz (sic) nice spacious rooms, honest breakfast included a little noise from the street.
To eat an incredible buffet The Grants Las Heras, Av. 1925 - Ciudad de Buenos Aires Tel: 4801-9099, so cheap for what you get.
Campo dei Fiori
Julián Álvarez 299 - more expensive than above but so good.
Cafe Tortoni Av De Mayo, a must.
Go to Tigre 30 minutes away by train.
A great website:
http://www.virtualtourist.com/
Have a good trip........