What is a good present to buy a couple in their 30s who has been living together for about 5 years and has everything?

In there 30's? Impossible for them to have already everything...
Now if I only knew how much you be willing to spend! Let's assume there is no limit, in that case I would send them to a Panama City Beach, Florida Vacation. Here is the link for reservation: http://www.vrbo.com/

I have something a little less expensive..

Ballroom dancing!  It is widely considered one of the most elegant skills to have. Whether they are interested in learning to ballroom dance as a couple, for a special occasion like for there wedding, this should get them in the mood. They can also just meet new people and learn a new skill; it is a beautiful thing to learn and watch...
Good luck!

Well I hope you enjoy these suggestions, they are not the average gifts. · Phoenix Arizona. Not a simulation. No flight experience required. Fly with real military fighter pilots in the world's highest performance certified aerobatic aircraft. Employ air combat tactics under the guidance of professional instructors while engaging the enemy. Introductory flights, advanced missions and multi-day adventures available. · picnic backpack Hosting a family picnic is as simple as filling our backpack with the family's favorite foods. The canvas pack is already loaded up with melamine plates, goblets, flatware and napkins for four, plus a bread knife, wooden cutting board, salt and pepper shakers and a sommelier's corkscrew. It even has a detachable wine cooler and sable-gray felt blanket. Exclusively from RedEnvelope. * padded.. for $79.00. · Hot Air Balloon Flight Discover the unforgettable thrill of soaring through the sky on a blissful hot air balloon flight and enjoy panoramic vistas across the landscape. Choose a gift certificate from the options below and make someone's dream come true. Or call us today on 1-866-987-9876 to book your place. With many locations to pick from starting at $229.00

I personally think you can't go wrong with cash or a give card. That way they can buy whatever it is they are needing. Not to mention you take the chance of buying them something they already have.

Other gifts ideas would include maybe an edible arrangement basket or a personalized gift.