I know, summer is the time to forget about school and have fun, but there are many advantages to taking summer classes. It's completely possible to be proactive and take a few summer classes and also make the most out of your summer.
You can get general education classes out of the way
Summer classes provide a great opportunity to get those pesky (and boring) general education classes out of the way. General education classes are difficult to focus on during the school year because you'd rather spend time focusing on classes pertaining to your major. Take one or two during the summer so you'll have the time to focus on them. (And you'll only have to focus on them for, like, a month rather than four months.)
You may get to graduate early
By taking summer classes, you're getting hours quicker than your original degree plan called for. By taking these classes during the summer, you could possibly graduate a semester or two early! Even if you don't get that many summer hours, you'll have a light load your last couple semesters, and who doesn't want that?
It may save you money in the end
Piggybacking off of the last point, if you finish your undergrad degree a semester or two early, any money or scholarships you've earned can be used toward graduate school if that's in your game plan. (I'm looking at the Louisiana kids enjoying eight semesters of TOPS money.) Use some of that rollover dough towards graduate school and save some $$$.
It'll keep you productive in between semesters
Taking a class or two throughout the summer will keep you productive and in school-mode, so the horrible "summer to fall semester" transition doesn't hit you so hard. Don't let your brains go to mush during the summer, y'all.
Do you *really* have anything better to do?
What else were you planning on doing with your entire summer? Think you can't travel if you're taking summer classes? There are plenty of classes that are only June or only July classes, so you'll have plenty of time to go wherever your heart desires (or watch whatever you want on Netflix).