Depending on your enclosure succulents could be a great choice. They do not require frequent watering (though they do require occasional deep watering) and they do not tolerate high humidity well (preferring 30-40% though some tolerate or even thrive in higher levels) and are adapted to prevent the release of moisture into the air, though, if it is a glass aquarium you are using without much side ventilation they may not appreciate the still air; that is one of the greatest challenges in a succulent (or cactus) terrarium. Also, they are also going to need a lot of light! But not direct sunlight as that would raise the temps in the tank beyond what they could tolerate (not to mention your poor gecko).... but a "grow light" or similar set 6" above the plants should prevent them from getting too "leggy".
I've had success in terrariums (as in, it's alive and growing, we're not winning any blue ribbons) with:
Echeveria runyonii
Aloes
Hen and Chicks (Sempervivums)
Sedum glaucophylum
Sedum morganianum
Sedum rupestre
...actually just about any sedum or sempervivum should work well for you, they are generally hardy and there are some hybrids out there bred to be even hardier! And easily found at Home Depot.
I'd also avoid red/reddish succulents as they tend to be a little more light-hungry and I'm not sure the fluorescent would really cut it for them.
Pot them in a mix that is roughly 30/40/20/10 of gravel/sand/compost/dirt... the mix will be mostly sand so you can "mulch" it with slate or large pebbles so your gecko can't get to it. Or you can grow them in loam which is a safe substrate for the gecko and requires less watering though growing them in pots is still probably the best place to start rather than directly in the terrarium (especially since it likely won't have sufficient drainage and succulents need that).
And pots don't have to be standard terra cotta! I've grown sedums in hollowed out driftwood.
I think that covers everything I've got on succulents.