Belize is a small country but still a big place with a whole lot to offer. In addition to traveling alone or with someone, it would be helpful to know if you want to just dive or also check out the mainland. And whether you're looking for a more Belizean type experience or just want to hang out at the hotel pool with the other tourists. Not making a value judgement here, just trying to determine what to recommend. It's your vacation and you should definitely do what you want to do.
Most divers head to Ambergris Caye. It has more of everything like accommodations, restaurants and dive ops. It's very well set up to receive tourists.
There are also lots of mostly high-end type resorts on the cayes (pronounced "keys" as in Key West), but a few more affordable spots, too. Very cool island experience.
Heading south, there's Hopkins. Hamanasi is very popular and enjoys an excellent reputation. They have their own dive op in-house. Hopkins is a good jumping off point if you're interested in jungle trekking, cave rafting, Maya ruins, etc.
Further south is Placencia, which is my favorite. It's transforming from a sleepy fishing village into another hot spot for divers and jungle trekkers. Some of the hotels have their own dive ops, like Roberts Grove and Turtle Inn. I always dive with Splash. They're by far the biggest dive shop in Placencia so you have lots of choices. And again, the jungle is just up the road.
The further south you go, the further the boat ride to the reef. AC is practically right on top of it. Hopkins and Placencia are about an hour boat ride away.
Since you'll be there for two weeks, you might want to think about splitting your time at two destinations so that you dive one week and jungle trek the other. A lot of people do that. The Cayo District is very popular for jungle trekkers. Lots of high end jungle resorts but more moderate places to stay as well.
It's easy to get around Belize by puddle jumper with either Maya or Tropic Airlines. No real difference between the two. Small planes, usually packed to the gills and most flights are a half hour or less. Great way to see the country from the air.