I want to plan a trip.....

Well.for starters.I wish you where my girlfriend. ;) joking.

Uhm. Well, what does he like to do the most. Is he an outdoorsy type of person? Music person?
$600 will get you basically many things instate and out but probably not out of the country together. Does he like cold weather or warm? I live in Georgia and I believe both states right now are on the hot side. I do know that Alaska is awesome this time of year around a constant 60-68 degrees.but I don't know how far $600 will get you there.

Best advice. Plan something that revolves around his likes even if it may conflict with something you might find more enjoyable; albeit, do try and find something you can both enjoy doing.being together.

I'd say a fly somewhere in the U.S. (you can get some great deals on tickets!) Have a romantic trip of your choice. Niagara Falls, Cave of the Winds, Mesa Verde, or Mt. Rushmore! Anywhere you want and this way you will probably still have quite a bit of many for dinners, treats, and gifts!