I find that Sea hem excel and prime are always good to have on hand. Carbon is useful or a spare lightbulb.
All good ideas, especially the prime and lightbulbs. Also, how about anything filter related (media, cartridges,etc), fish food, nighttime LEDs, testing supplies. Their prices are usually always better than what I can find around here, so I would suggest you get things you know you will need in the future.
Also, if you have other pets, you can buy their supplies too, it all comes in the same package. You do not have to stick to one type of animal. I get packages that contain fish, dog and cat stuff.