I took my 2 year old and 4 year old to Costa Rica last month.
They enjoyed CR just as much as they did Belize and Puerto Rico.
We stayed on Flamingo Beach - What age kid doesn't love the beach and a nice hotel pool?
We spent a lot of time close to the beach and pool, but we took two all-day drives:
We swam in waterfalls - What age wouldn't love to swim in a pool at the base of a waterfall?
They loved the volcanoes & the cloud forest.
We saw monkeys in the trees on the side of the road - What age kid wouldn't love that?
Like all kids they are going to take away memories that are linked to their age and what they think is great might be different from you.