Do you have a place to stay yet? That would be the first thing I would do if staying the Maho area. Dinner reservations at most places aren't really necessary, but could be helpful at some of the nicer restaurants such as Sale & Pepe. You might to check Trip Advisor and see the restaurant rankings for both the Maho and Simpson Bay areas. Some places will have links to Open Table which will allow you to make dinner reservations in advance at a preferable time. Given that this is Christmas week I would probably move in that direction.

As for the beaches with chairs, lunchtime dining, etc. there's no need to reserve anything in advance IMO. You'll be getting to Maho beach plenty early for that, but I would suggest taking the short walk by the golf course and spend time at Mullet Bay Beach which is typically less windy and has some nice lunch options (such as Rosie's). Simpson Bay Beach and Kim Sha are also nice beaches within walking distance in the other direction.

As for PInel, I've never paddled over there but there is a small ferry boat that will take you there and back, and it's really cheap. But, once again no need to arrange that ahead of time. If you want to rent a kayak, however and do this yourself maybe someone else on this board can chime in with some suggestions. Hope this helps.