I would relax and do something fun! If you wanted to stay fresh, i would browse journal articles in your subject of interest...you will most likely be reading scholarly articles rather than texts. You should still have access to your online database from undergrad (PsycINFO, science direct, PsycARTICLES, etc..)
If you really want to be an overachiever, you can familiarize yourself with SPSS and stats in general...You will need to learn what statistical analysis is appropriate for certain kinds of data sets. Also, concepts such as validity and reliability...maybe check out "Standards"...
Good Luck..