I wouldn't do Ambergris or Caulker for a beach vacation; the focus there is taking boats out to the reef to snorkel and dive. The beaches around town are mostly hardpan (tho some import sand), and the water has lots of seagrass (and docks and boats). Can't think of anything "cute" in those places.
Placencia is a much better bet, but check out your hotel carefully, if beach bathing/swimming are your aim.

http://www.belizelogue.com/beaches-of-belize-probably-not-what-you-expect.html

I'd focus on Ambergris but also take a look at Placencia. Our beaches are fine but not stellar and better at some hotels and areas than others. Plenty of nice accomodating places to stay and you'll easily find one that matches your needs and budget

I prefer the beaches on Ambergris. The sand is softer and whiter, and the water is clear, Caribbean blue. They are narrow, and, especially in town, there are lots of docks.

There is also sea grass, it's the price of having the barrier reef less than a mile from shore. There are few waves, hence, the grass.

Placencia has postcard pretty beaches, but, at least in the village, the sand is darker and coarse. On my few trips there, the water hasn't been clear, nor was it blue.

What kind of budget are you looking at for lodging?

We stayed at Victoria House on Ambergris and loved it. The beach there is big and has beautiful soft, white sand and the water is gorgeous. They have a restaurant and a bar that both offer dining. Also drink service etc. around the pool. There is sea grass in the water, so we did our swimming at the pool. Lots of hammocks and lounge chairs around, beautiful palm trees. We had the place to ourselves a lot of the time. We chose Ambergris due to its proximity to the reef and all the restaurant options in town. You can either bike to town, take a golf cart, or take a taxi for about five bucks. If it's in your budget, I'd go to VH.

I think that there is a tendency here to suggest favorites over what the OP mentioned: "a nice beach vacation? Sun, swimming,"
The intent seems to be beach swimming, and she's offered pool, and the narrowness of the beach is mentioned. Yes, some resorts have improved their beaches, but are really not what OP seems to be looking for.
However, there is more variety on Ambergris for lodging and dining.

The place I suggested has a gorgeous beach, great drinks, relaxing, sun, and swimming at the pool. Ambergris also has a nice selection of great restaurants, bars, and the other things the OP was looking for. If someone goes somewhere and has a great experience then it seems logical that they would suggest it to other people. We spent a lot of time on the beach there and loved it. The snorkeling at Hol Chan is also amazing, and we loved swimming there. I'd love to go back.

I stand by my comments as well. I simply don't care for the beaches in Placencia. The sand hurts my feet, the water is dark and murky.

Ambergris, though the beaches are narrow, has soft white sand and beautiful Caribbean blue water.

OK, my last 2 cents; the sand is soft where it's been created at resorts (I say this after miles of beach walking), ditto the water for swimming (not so great due to sea grass, muddy substrate, and constant boats).
I liked it, but found the snorkeling so-so (Hol Chan was Grand Central Station, and not nearly as good as other places we've been), the "beach relax" vibe ok IF you are staying in a resort, and the beach swimming almost nonexistent.
(Viewing the actual resort sites will give an idea of size of beach.)
For lying in a hammock or beach lounge and watching water and people go by? Great. But, not so great if you are thinking of ocean swimming from the beach; it's all boat-excursion oriented.

Reading the OP, I see "girls getaway for three girls in our early 30s" and "We also really like restaurants, shops, cute bars/cafes" and "relaxing with frozen drinks"....all of that adds up to Ambergris in my experience. Much more to do there. The beaches are nice and I get the sense that the OP is using the term beach vacation as a general description of an island holiday.

We stayed in a lovely condo right next door to Victoria House. It was amazing. There are many places closer to town that are equally as nice. Katie is better to advice on which resort is best.

AC is a coral reef so it is made up of coral and coral sand, no hard pan. You can see the beach very well on gmaps and youtube so look at it there. swimming is often done from the resort dock as the beach can be narrow and have seagrass.