Some places that offer both and one to consider non-zipline.
Belize as mentioned - they also have rivr tubing, mayan ruins, the jaguar preserve, good barrier reef diving. I'm not sure where the ziplines are in relation to the diving - think they're inland so you might have to split your trip. Southwest recently started flying to Belize City - from there it's a cheap ferry to Ambergris Cay or a $200ish r/t flight to south Belize which puts you close to the other activities but diving is further out by boat.
Roatan has both.See what you think of Anthony's Key dive resort as they have a BOGO so the whole stay for both of you is around $2200 - flights being about $800 more r/t pp. It's also where you can dive with the Dolphins for around $125/pp.
The beaches mostly suck on Roatan except for West Bay which is also where one of the ziplines is. Resorts in all price ranges in the area. Lots of bugs on Roatan if it's an issue - probably not a bad idea to the the Malaria program..
There's also St. Thomas.- they even have a tandem zipline. Personally we liked the East End more than downtown. There's several nicer resorts with on-site dive shops - Coki Beach is a decent shore dive and from there you can also "drive" (car ferry) over to St John. Good shallower diving in that area in the cays.
Also you can take an hour ferry ride to the BVI's - see the famous Baths. Very picturesque. The fast ferries run so often it's actually possible to dive with operators in the area - DiveBVI for example is about a block from the ferry port on Virgin Gorda. Once a week they dive the RMS Rhone - a massive shallow wreck.
Or split your time between the two countries - we did.The ferries make it easy - all flights to both countries are thru St. Thomas and one of the ferry ports 10mins. from the airport. The BVI's are a little pricier. What's nice is they have better than expected restaurants given the small population since there's a lot of big villas.
Downside right now is both St John and parts of the BVI's took a massive hurricane hit last year so some things may not be open yet.
If you want to forego the ziplines - look at Curacao. There's resorts in all price ranges - IMO better shore diving than Bonaire and many of the sites are easy beach entries with dive shops on site. That doesn't exist on Bonaire except at the dive resorts in town.You should be able to easily do Curacao for well less than your budget -
www.curacao.com
It has all the things Bonaire lacks - nightlife, casinos, good restaurants, the Ostrich Farm, Sea Aquarium (they do Dolphin dives there) etc.