You will need a valid passport to travel to Argentina. No visa is required for US or UK passengers, but other nationalities should check with their local consulate.

4/1/16 As of today there is a suspension of reciprocity fees for U.S. passport holders visiting Argentina for stays of 90 days or less. Here is the official press release from the Argentine Embassy in the United States:

In order to strengthen the relationship between Argentina and the United States of America, the Argentine Government has resolved to suspend the collection of the reciprocity fee from U.S. passport holders who visit our country for less than 90 days, for tourist or business purposes.