Thanks, all. I'll check out some of those options you mentioned.
Ambergris Caye would be in August/September and I've read some horrible things about mosquitos that time of year. Plus I read not many actual beaches and that the scuba diving (aside from the Blue Hole) isn't that great. It's hard to know what to believe!
Mosquitoes can indeed be a challenge anywhere in the Caribbean nearly, plus noseeums on Roatan beaches; just part of being a Caribbean diver: take two cans of DEET spray - one for the room, one for the dive locker.
Blue Hole is a
"did-it" dive and the few minutes at the stalactites is interesting, but newbies shouldn't be going to 150 ft. It happens everyday, but shouldn't. On the other hand, there
is some nice diving there, and nice beaches.
We've only done one other cruise (to Cozumel and Grand Cayman, and of COURSE we didn't get out scuba certification ahead of time - it was for our honeymoon). Now I find out those are two of the best places to dive. As for what we're looking for - being able to dive a few times, not all day every day. We can't afford that. Beautiful beaches to chill out on, and some restaurants, etc. nearby. The husband likes some adventure-type stuff as well - so having other things besides diving available would be helpful.
Getting to and staying at your destination for a week is most of your expenses. Diving is not that much more. Many like to do a 2 tank trip in the morning, then other ideas afternoons tho. Other interests could have some bearing on the destination tho.
Someone told me Bonaire, but $1,700/per person plane tickets won't allow that either

We're looking at $4,000 total trip if possible.
I'm afraid your source was badly misunderstood or clueless. OKC-BON in July runs $894 RT each, from Tulsa $926, cheaper in August - $801 & up. Beyond that, I can't tell you much about Bonaire, as those fares, the style of diving, and some of the local challenges with required pickup rentals and occasional burglaries discouraged me. Bonaire fans will tell you otherwise, but I just haven't been.
My fav remains Cozumel so I can tell you more about that if interested, including cheapest ways to get there and stay on a nice beach. Like air & hotel combined at a nice beach hotel in late August for $706 each taxes included, and up depending on hotels. But you might prefer another destination and I'll try to help with air & hotel if you like.