Teeth: Get a water additive and/or some Greenies treats; the more you take care of their teeth, the less likely you'll have to get an expensive teeth cleaning. I usually just put in three greenies or so in their breakast everyday/
Scratching post
Get one of those "cat condos"; they don't always like getting inside them, but it makes a great "extra" scratching post, plus my cats like sleeping on top
Cat bed; again, no guarantee they'll use it a lot, but a couple of my cats like napping in it, one in particular uses it all night if he doesn't feel like sleeping with me
Toy mice: get some of those packages where it has a bunch of little mice you can throw, preferably the well made ones, NOT Hartz; they have cheap cardboard tails which tend to wear off realtively easily, and you don't want them swallowing crap
Teaser Wand: Cats LOVE this. Don't leave it out on the floor, only have it out when you're actually playing with them. Preferably get one with a soft toy on the end instead of a hard golf ball or something. Again, you don't want to give them the opportunity to rip off the feathers and eat them, but they sometimes like grabbing the toy in their mouth and running away with it. Hee.
Cat carrier: If you don't already have one, try to get one that's got an open grill on top. It can be easier to get them in and out from the top, plus it gives them plenty of air, so you don't have to worry about the side vents being restrictive. Also get some soft towels to put in there to make it more comfortable for them when in travel. Get some shelf liner from Wal-mart, a hardware store, or even one of those $1 stores. Put it under the towel to keep it from sliding it around too much.
Catnip IF you choose to use this, wait until they're of adult age, it can hurt them if they're still kittens or still growing. you can add it to their food to aid in digestion
litter boxes Get a "clamshell" style box that has a roof on it. If they tend to "sling" the litter around, this will help keep it inside. Also, get one of those litter-box mats to put in front. Any excess litter on their feet should get caught in that instead of getting on the carpet. Clean them out at least bi-weekly. As in, empty all of the litter and washing it out with soap and water, maybe some diluted bleach. Make sure the bleach smell is gone before you put it back. Preferably clean the top half of the litterbox too; they can sometimes get crap on there, believe it or not. Consider getting two boxes. It's not necessary, but if case you need to be gone overnight or don't have time to change it before leaving in the morning, they don't have to wallow in last night's filth. Try to use a scooper on it twice a day. Get a METAL scooper. The plastic one I used evetually broke; some of the waste can get pretty heavy, so make sure the trashbag doesn't get so heavy it breaks when you get ready to throw it out. Don't bother using litterbox liners; they suck and tear fairly easily.