Just want to clarify a few things...you say trip of 3-5 days. Does that mean you want a diving centric trip? If yes, you will want something that will get you there quickly. LOL Do you want boat or shore diving (or don't care). Do you want to cook your own meals (aka apartment) or do AI or resort area.
You are in PNW, so am I, so I know flight times approx. to some destinations. You can get round trip to/from Seattle to Cayman (GCM) for approx 8 hours...if you pay for the better flights. leave first thing in the morning, on island around 3pm and start diving. Obviously pacific coast will be faster to get to (Baja etc) and I know nothing about the pacific coast mexico area.
Considerations: Caymans- Unless you stay at Cobalt Coast or Sunset house, you will be limited to 2 boat dives daily without renting a car such as if you do a package with Amabassador divers and Sunshine suites. What do you want to do? boat and/or shore diving? Then go CC or Sunset....or rent a car and go to many local dive ops. You can pay about $300 for car rental for 5 days. Then do a VRBO for a place to stay (don't get hit by the single supplement additional $$). So...air approx. 800, car 300, lodging 600 (this is a wild guess), diving (7-10 per shorediving tank) maybe $150 over 4 days if you dive as much as I do, plus food. That brings you in under budget for 4 days of diving, 1 day deco.
Roatan-Anthonys key/Fantasy Island will get you in under budget..but flights seem brutal (16 hours travel each way so if your time is short then I would leave this one off?). We head for CocoView/Roatan for the first time in 22 days. Not counting or anything.
Bahamas-some decent fly times and great flight prices. Never been there. Yet. The year is still young
Belize-Location you picked will have LONG travel time before you can hit the water and limit your dive time. If I recall correctly.
Costa Rica-Heard not great viz...but maybe you want to do more than just dive?
Have you considered Curacao? I was trying to stick with your list (as you only listed place you had not been..but wasn't certain you considered Curacao/had already been there. Same travel times as Roatan or shorter. You can stay at All West Apartments, unlimited shore diving, truck rental included for about $800 PP per week for 2 people. So could be about $700 per 5 days for 1 person? No idea. Epic house reef. Great dive op, super cold air conditioning and less touristy. You get Bonaire reefs with Hawaii sand beaches
We have only been 2x but both have been awesome.
Good luck.