research is cool and all, but it can be done concomitantly with m2-m4; don't waste what will (effectively) be your last summer vacation in a lab...
i like the idea about spanish, it's terribly useful and relatively easy to get the basics down... plus summer classes at most colleges are more concentrated, and allow lots of the day to be used for fun. d=)
or... do something fun and wholly non-medical... cultivate a talent for glassblowing, or learn about the intricacies of curling, coach a local kiddo sports team, etc. ...
seriously... being able to have something "eye catching" on your CV will likely allow interviewers to attach a name to a face to a file. plus, having somehting to do outside of medicine is always the best way to stay sane, and PD's like it. Shem wasn't long off when his character "ran for fitness and fished for calm". d=)
just my $0.02 (actual value 1/20th of $0.01)
-t,md