Catsan Cat litter is very good, its expensive but does not smell as much as the cheap versions. (dont go for the clumping catsan litter.)
You also can buy cat litter trays with hoods over them and incorprated in the lids of some is charcoal sheets. Cleaning out regulary helps. Diet as really no play in smelly cat litter trays.
With the Catsan cat litter, when they open their open their bowels, you can scoop it up with a poop scoop, gently tap the poop scoop inside the tray till excess litter is dropped through, then flush the poop down the loo. People who come to visit me, wouldnt tink i own 3 indoor cats as it never smells of cats. ( they all had the snip anyway.)
If you been using the clay/clumping cat litter, that causes lot of smells of cats. Here is what i use.Its expesnive but does last longer than the cheaper versions.