We will spend 4 days at Divi Little Bay at the end of April. This is our first time in SXM and I am wondering what there is to do close to the Divi and if the restaurants/bars at the Divi are good. We mostly want to read/relax/nap/drink cocktails. We are considering a car rental, so could travel a bit for places that are worthwhile. I would also be curious to know the best place to rent a car from, I've read some of the shady stories and would like to avoid that scenario, if possible. Thanks!

Divi is a little isolated from a lot of places, so you definitely should rent a car, yes. As far as a car rental recommendation, everyone has their favorites. Just check the TTOL sponsors list, send them a request for a reservation and go with whoever you choose.

Yes,a car will be useful for Little Bay. Restaurants on site are OK for beach lunches but in the past have not been great for dinners. I understand they have made some changes but haven't got any personal experience yet to report. But you wouldn't ever want to eat there for all meals, given all the wonderful options on the island.
Thrifty has a rental car location right at Divi, so depending on your plans you could rent a car there. That could be a good idea for example if you are arriving later in the day since you're a first timer. We've used Thrifty there and the cars were fine and rate seemed comparable to some others we've used.

If you want to rent for just a couple of days Divi has a car rental on site.