Placencia is a great, quiet fishing village, I stayed there for two weeks just about 4 years ago. Costa Rica is fine too, I stayed in Playa Hermosa (near Liberia, on the Pacific side) 2 years ago. Costa Rica is definitely more developed, I prefer the fishing village feel of Placencia though. Different strokes for different folks.
Been to Belize a couple years back, all the stuff you mentioned was fun, zip lining cave tubing all that stuff snorkeling was okay "but i lived in hawaii so im bias that" The most fun we had was just driving around exploring if you can get up to Orange Walk it's really cool area actually got a lot of Amish folks from Pennsylvania there. If you can get over to Guatemala the Mayan ruins there are incredible they're pretty much just in the woods un regulated, do whatever you want walk around onem, crawl on them, take them home, jump off, anything goes. And Guatemala is cheap, it's like a true 7 to one exchange rate. We stayed in a lakeside Villa that had manicured grounds like the Four Seasons included dinner & bar and it was like $25 us a night for four of us.
Our activities are pretty regimented due to it being a “tour”. Won’t be any time to venture out on our own,
Surely you can find time for an after hour all-night hooker & blow session that ends in a happy ending.
Belize is fun and beautiful. Tons of stuff to do since lots of tourists frequent as it's a diving mecca. If you have a free day, look at taking the sea shuttle over to Roatan. Check out any place besides where the cruise ships dock in the northeast corner of the island. I'm partial to the coast about 3 miles south west from where the ships dock near the West End. Locals, art, cheap food and booze, lower cost kayak/sea sport rentals, clean and clear beaches with stunning snorkeling. Bring USD. Take the sea shuttle back to Belize at the end of the day. Have fun, stay safe.