well, I have to agree that leos are the easiest. You can feed an exclusive diet of mealworms, which can be kept in the fridge, and they are a very forgiving species when it comes to incorrect husbandry. as long as they are heated, fed and watered, then they will be fat and healthy.
crested geckos are another good beginner species, however they are not AS easy as leopard geckos. as nanihokiahi said, they need decently high humidity [70-80%] which DOES mean misting the cage daily, they are also arboreal so they need vertical space, but not only that, they need a densely planted viv. if the geckos hang out on the glass all day long, they get something called "floppy tail syndrome" which is when the weight of the tail becomes too much, and the pelvis twists all crazy. it doesnt hurt them, or keep them from breeding, but it isnt exactly ideal, or astheticly please. another thing about cresteds, is that they sometimes drop their tails [which feaks ME out] and they WONT ever grow back.
as for feeding them, t-rex makes a complete "crested gecko diet" which DOENST need supplements or live foods. if you give crickets, they will eat it, but its not required. it does give a tiny bit of added protien, which is important when dealing with growing hatchlings and juveniles, but you can also feed the "Gargoyle gecko diet" versian, that has 30% protien as opposed to 15-20% that the CGD has. no ill effects will be had if you feed generations of cresteds exclusively CGD.
If your room temperature stays in the high 60s to low 80s they dont NEED heat, however, basic knowledge of reptiles tells you that cold blooded animals fare better when given an opportunity to thermoregulate with a temperature gradient.
leopard geckos are also more easily handled, crested geckos JUMP which can sometimes be hard to get used to, but you eventually get to predict their moves over time, even still, if a gecko jumps to the floor from your hands if youre standing up, thats a far fall, and they can get damaged. leopard geckos are kindof spastic but most reptiles calm down with regular handling [crested included] and tend to be more content to just sit there and do nothing.
crested geckos can only tolerate short bursts of handling, and when dealing with children, I tell parents that they should be treated like a fish tank, something to look at, not something to touch.
both cresteds and leos come in a variety of colors and morphs, the leopard gecko morphs are genetic most of the time, and cresteds are just... you breed 2 together, and you get a mix of both [kindof like people]
Might I suggest, that whatever species they decide on, have them pick up a book. there are 2 fantastic books out right now. the first is the Rhacodactylus book, which covers every species in the genus. the second is the Herpetoculture of Leopard Geckos, which goes in to GREAT detail of every aspect of care, from set ups, breeding, morphs and genetics, both books are great for the beginner or the seasoned keeper.
If you really want to win some points with your boss, I usually have a spare male of either species laying around
