Reptile carpet/paper towel/tile (possibly coconut fiber)
Avoid aspen, sand, calci-sand and walnut shells.
Leopard gecko substrate is where things get spicy. There are many arguments over whether sand is an applicable selection of bedding for a leopard gecko, along with many other potential options including coconut fiber, aspen, and more.
I highly suggest you use a non-loose substrate, meaning it cannot be ingested, reducing all chances of impaction, which we get to later. Reptile carpet, paper towel, newspaper and tile are the three most common responsible options.
Reptile carpet arguably looks the nicest, but is a bit more difficult to clean. It requires actual washing, as it's a fabric material which absorbs liquids like urine, and fecal matter may stick to the carpet. This is still my substrate of choice with my geckos I keep permanently.
Tile also looks quite nice, is heavy, and slightly easier to clean. Just wipe or scrub tiles off when they become dirty.
Paper options look a bit more... DIY, but are by far the most sanitary and simple option. When they become soiled, simply toss and swap it out! This is what I use with almost all temporary leopard geckos I keep, because of the reasons mentioned before.
Coconut fiber is a loose option that's frequently used. It looks quite nice, and does the job. It rarely causes impaction, unlike sand. However, when dry, it frequently gets very dusty. The dust created by coconut fiber can get messy, with a risk of inhalation from your leopard gecko. It's hard to argue against coconut fiber, as many people keep their dry gecko species on it with no issues, but it's personally on my list of substrates to avoid when not being used with moderate to tropical species.
Aspen is something I don't have much experience or research on. Although I've used it, I quickly stopped doing so with leopard geckos, as many people have reported their stomachs being scratched or cut by the chips or shavings. Although most reptiles do well on it, leopard geckos have especially soft and delicate bellies.
So, sand. Why not? Every pet store suggests it, right? Unlike soil, sand is made from quartz/rock, and is quite hard for stomach acids to break down and pass. As leopard geckos live their life, they tend to lick everything along the way. Even if you feed your leopard gecko outside of the enclosure/in a bowl, they lick everything including decor, people, other geckos, and yes, the ground. This leaves a decent chance of your leopard gecko ingesting a couple pieces of sand here and there. Sounds harmless, however, because it often ends up clogged in the gut of animals, it can build up overtime, and eventually result in an impacted animal - filling up all space for food to digest, absorb, and excrete from your gecko. Unfortunately, based on all sources and people I've talked to, it's hard to spot impaction before it's too late, with surgery being expensive and with no guarantee of saving your animal.
Many people say that "If your leopard gecko husbandry is 100% perfect, you never have to worry about impaction. They will be healthy enough to pass all substrates." What is 'perfect' husbandry? Why have so many leopard geckos that appear perfeclty healthy been killed by such a simple issue? And is sand really that great?
Let's say a leopard gecko can live its life perfectly fine with husbandry that's 90% perfect. (Again, the definition of "perfect" is never the same). The only thing your gecko apparently cannot do with this level of perfection is pass sand, while everything else is fine. In this case, is it really worth attempting to achieve 100% perfect husbandry? Well, yes, we should always hope to improve. But, because this is so unclear, and sand is such an underwhelming item to take a risk for, I believe you should simply ignore it, and move along to a responsible option.
Don't get me started on Calci-sand. Your gecko cannot digest it, it is not healthy, it is a product no gecko deserves to suffer on.