Devin pretty much answered all your questions. I might add that while darts don't deliberately go swimming, a lot of them do appreciate shallow waterways to splash around in and hunt, pools filled with smooth rocks, as well as waterfalls to climb. Designing and building these is an entirely different subject, and I think a search here will bring up some websites and information. They can be high tech or very simple. Keep in mind that the secret to preventing rot is good drainage of whatever substrate you decide to use. Your terrarium should either have a false bottom, (if you build waterways and falls,) or a layer of sharp rock and gravel with a barrier between this and the substrate, from which drainage water can be siphoned off. (Mechanically or simply with a small aquarium airline tube.) The barrier between your substrate and drainage area can be as simple as some cocoanut fiber matting (Welcome mat grade), perforated plastic garden weed control sheets, or discoveries of your own.
You might also consider that most darts will use all layers of a terrarium, from top to bottom, depending upon how it's designed. We're talking only a couple of feet here between terrestrial and arboreal, while in a rainforest, the difference is many meters. I was surprised to find that my pumilios spend a lot of time on the very bottom of the tank, and that my leucomelas sit in the highest part of theirs in bromeliad leaves when they're resting.
At any rate, I hope this gives you a bit of a start for some great fun with some delightful frogs.
For instance, these Dendrobates galactonotus are not supposed to be way up there in the bromeliad leaves, because they are ground dwellers. Someone forgot to tell them this.

-----
Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus