You can put all kinds of things in a litter tray, but Chinchilla sand works well for soaking up the pee and stopping it smell too much. You could use a different kind of substrate in it or just some of your usual substrate as Cypher says, but I think the litter tray would need cleaning out more often in that case. I did used to use the potty litter. Our hamster tried eating it when I first put it out, but spat it out again - he didn't try again after that, so I carried on using it for a while, but in the end I switched to Chinchilla sand as he would kick some out of the litter tray and it would get mixed in with his hoard and I thought he might accidentally eat some (if he wants to eat sand with his hoard that is up to him! I think actually he rarely eats that hoard and just hoards it but I have more peace of mind now I'm using the Chinchilla sand. It's not cheap but one of Supreme cartons lasts me a month to six weeks using it in his potty (he won't use a sand bath - kicks the sand out and replaces it with bedding lol. He tolerates the sand in his litter tray though.
I just had a look on Amazon.com and the Supreme one looks quite expensive in the US. Seems to be about $15 including postage - unless you know somewhere you can buy it cheaper. There is a Kaytee one but reviewers say it is too dusty.
Amazon.com : Supreme Petfoods Science Chinchilla Bathing Sand : Petsuppliesmisc : Pet Supplies
Some people use children's play sand but it needs baking I think.
You could try something like this paper-based unscented cat litter (Yesterday's news) - it looks very similar to the paper pellet one made by Finacard in the Uk. It's just compressed paper - non clumping. Generally cat litters aren't safe but this one looks ok and as it's a big bag it should last for ages.
Amazon.com : Yesterday's News Cat Litter, Non-Clumping, Unscented, 30-Pound Bag, Pack of 1 : Pet Litter : Pet Supplies