I'm planning a 5 night getaway, no kids, for my husband's birthday and our anniversary. Both are "round" numbers, so kinda big deals.

Requirements:

1) Not flooded with tourists;
2) Good beach very close, if not attached to whatever hotel we choose;
3) 5-6 hour non-stop flight max;
4) Budget is $5k total, including airfare and hotel;
5) Bonus points for interesting, non-beachy stuff nearby.

I've been looking at Panama, Cartagena, Colombia, Puerto Rico, Tulum.

Any ideas/recommendations?

we just got back from cabo and it was great. the beach by the resort wasn't suitable for swimming due to the currents, but it's still beautiful to sit in front of.

Ambergris Caye or Caye Caulker, Belize

Cabarete or Bayahibe, Dominican Republic

Coco Plum Resort Belize checks your boxes.

Huatulco, Oaxaca. We were there in early December and were supposed to go last week but cancelled due the stupid blizzard. We're going in March. Even less crowded now that the Canucks are grounded.

Huatulco is absolutely beautiful and very few tourists, but Cabo has more amenities. We have been to both recently.

Grand Cayman?

Costa Rica?

Dominican Republic will be a great option for your budget, non-stop flight and great beaches.
Could get a little crowded/touristy but there are some amazing, tucked-away, private resorts you could stay at that would help with that.

Coco Plum Resort in Belize is a great option as someone else mentioned...Mahogany Bay in Belize is a favorite too.

I like Huatulco too...Riviera Nayarit would be nice.

Tulum and Akumal (both south of Playa del Carmen) have some really awesome boutique resorts with charm.
Viceroy Riviera Maya comes to mind also...

I've had some requests for Puerto Rico lately and they are really starting to intrigue me more - Serafina Beach Hotel and Fairmont El San Juan are a popular choice.

Le Blanc Cabo !

We stayed at Hotel Esencia for a milestone anniversary. It is between Playa del Carmen and Tulum - boutique hotel on the grounds of what was originally a private estate (original fancy house still intact and beautiful).

I also love the Turtle Inn in Placencia (Belize) for a special occasion. It is lovely and has great beach front villas.

5-6 flight from DFW includes pretty much every beach in the continental US, Caribbean, Mexico and central America.