All good advice so far. I stayed at the Sonesta Maho Beach for a week in February – it's a decent hotel and a stone's throw from the beach itself. Not sure what room you've booked, but we managed to get one with a beach / runway view (apparently it's a common request). Worth asking the question.
I wouldn't bother with a taxi from the airport, it's an easy 15 minute walk, or there are regular minibuses running past the front door. As regards spending money, from memory the sun loungers at the Sunset beach bar were around $10 each (aim to arrive by around 10am, as they go quickly) and topless girls drink for free at the bar, so if you're taking your better half and she's so inclined, it makes for a cheap day!
I would also thoroughly recommend a day trip to Statia (St. Eustatius) on a Winair twotter. You'll fly via Saba, where the view through the flight deck doorway is breathtaking. Statia is a different world to St. Maarten, and great for hiking, snorkelling or just pottering around.
While on St. Maarten, it's well worth hiring a car and spending a day or two exploring the French side of the island, which is quieter and less gaudy than the Dutch half. Marigot, the capital, is a nice little place and around half an hour's drive from Maho Beach. Another half hour (depending on traffic) will get you to Orient Bay, which is absolutely stunning.
Just to the north of Orient Bay is Cul de Sac, where for around $6 you can take a 10 minute boat to Isle Pinel. This is uninhabited apart from a couple of beach bars / cafes and lots of sun loungers, and is a great place to spend a quiet afternoon away from the crowds. It also has a lovely shallow beach with water that's as calm and clear as a paddling-pool.
I'd better stop now as I'm getting depressed – wish I could go with you!!