Montezuma would be a great choice, I've met lots of women traveling alone there and never heard any complaints. I think it's a particularly extra safe area, don't leave valuables out alone when you are swimming and you'll be fine. I've heard very few theft stories there (I've been there 10+ times), but never heard any violence issues.
You said you're a beach person, but don't skip the waterfalls, I go every day when I'm there. Some people save it for later in the trip and then get sad there isn't more time to go back to the falls.
Also, be sure to do the 2 hour hike up the beach to the other smaller falls.
The snorkeling Tortuga Island tour is nice. Not an absolute must see but nice. Wait until the morning to get your ticket so that you know it'll be a nice day, on the off chance it's sold out, you can always go another day. It's the only place I've ever snorkeled (lots of fish, the coral is okay) but the boat ride is always great.
I'm going to recommend my favorite hotel there, the Montezuma Pacific (Montezuma Pacifico) hotel, I just updated my review so you won't find mine until they approve it. It's inexpensive and the staff is awesome and protective (they will probably tell you safety tips and something like you aren't allowed to bring men back to your room). I've been there 7 times for about 50 days total. A/C, wifi, refrigerator, large digital safe (in the room) hot water, shared balconies and some rooms have TVs. (It's not on the beach though, I'm not sure what you are going to find that's a bungalow on the beach that isn't a lot. Ylang Ylang is lovely, but I can stay at Montezuma Pacific for a week for the price of a night or two there.) Town is very small and it's nice to be in the center of it (but off the busy strip).
There is a place that's bungalow-ish, but no A/C and they aren't any much closer to the water (and I think they're more expensive). The name escapes me.
Here is my recommended item for people traveling to the beach (or waterfalls) alone. It's a wired mesh security backpack that you can lock to a tree, seriously. It's great for locking up your cameras and/or phone while you go swimming.
You'll love Montezuma. I'm not sure how you are getting there. You don't really need a car. Montezuma Expeditions can book you a shuttle ($40-45) from hotel to hotel from San Jose (or probably wherever you are) but you usually need to manage your luggage for the ferry ride. If you happen to be coming from San Jose proper, you can catch the directo at 6 am and 2 pm from La Coca-Cola bus station for $12 (or $14), that bus has A/C and they lock up the luggage for the ferry ride (you get a number ticket and you can't get our bag without it, makes me feel secure).