It wasn't too long ago that summer meant tv reruns. As a tv junkie, that was always a bummer. I would finally have time (no homework, no grading) but I'd seen what I wanted to see. Then cable channels started showing first-run shows, and I was hooked. The channel I watch most often during the summer (some of the shows air during the winter months as well) is USA Network. They have what I've seen called "Blue Sky" shows. They are suspenseful, fun, and light, great escapism television. It started with Monk about an OCD detective (sadly, this show has ended). Now, I regularly watch Burn Notice (an ex-spy helping the less fortunate in Miami - it's the knight-errant theme, like Magnum P.I.), White Collar (a con-man helps the FBI solve white collar crimes - pretty cool ideas), Covert Affairs (young CIA operative, making her way in the business), and my favorite current show, Psych (a fake psychic detective - I can't recommend this one highly enough; the Mentalist is a total rip-off, but Psych has great chemistry between the leads and can be laugh-out-loud funny at times). Watch it!
TNT has Leverage, which is Ocean's Eleven meets Robin Hood. The cons and plans are well done, and I never guess right about what's going to happen.
Finally, SyFy has Warehouse 13 (Indiana Jones meets FBI meets wacky gadgets - watch a few shows and it becomes addicting; they use lots of fun ideas, like Poe's quill or HG Well's time machine), and Haven, which spun from a Stephen King story but actually has nothing to do with the story except in passing; lots of strange, possibly supernatural or superpowered goings-on being investigated by an FBI agent who has a knack for attracting such things. Haven is not nearly as intense as The X-Files, but has some of the same ideas.
If you can only see a couple, watch Psych (watch it already, and pay attention to the pop culture references, particularly from the 80's) and Leverage.