I would say yes to the rental car, but don't leave any valuables inside! As for being safe, you'll be fine. I wouldn't wander around the French Quarter or Dutch Quarter after dark, but otherwise I have never felt 'unsafe' here. Just use common sense. As for things to do there were more before Hurricane Irma, but many nice options still remain. Parrotville is a blast, assuming you like these colorful pet birds. Another fun day trip is a visit to Maho Beach and watching the planes land over your head. When you get tired of that, head to the Sunset Beach Bar and Restaurant where they have good food and good drink. And, my personal favorite has always been Orient Beach. Some of the beach bars have been rebuilt, and for $30 or so you can rent two beach loungers and an umbrella. Walking Orient Beach from top to bottom is also great way to get exercise, but you may (will) encounter nude sunbathers at the southern end of this beach (where the former Club Orient once stood). Of course, if you give this a try (like I did many years ago) you might like the 'au natural' approach!

Unfortunately, some of my favorite things from the past are gone (i.e., The Butterfly Farm), but St. Martin still has much to offer. It's the best Caribbean Island IMO, and far less expensive than neighboring St. Barth's. Just leave your cares behind and have fun here!