This is similar to the packet I provide students who will be taking AP Spanish the following school year. I typically add in my email address and additional information about the materials they should keep from previous levels.
In this introductory letter (available in both Spanish and English), I provide a description of the AP curriculum and assign 2 activities to be done over the summer. Students are to study the Spanish speaking countries and their capitals (I provide a blank map here and generally a completed one for students, as well) for a quiz the first day of school. Also, they are to locate 3 news articles related to one of the AP themes and a Spanish-speaking country.
This is a nice way to introduce the AP curriculum and expectations.