What to put in a spider cage?

It depends on the requirements of the individual species. Remember - you're trying to match their natural environment. This will include suitable substrate, hides big enough to conceal the spider from prying eyes, possibly branches to climb on (arboreal species), a water dish, a heat mat & thermostat (if you cannot match their temperature range naturally)... There are plenty of books available - because tarantula-keeping is becoming more and more popular.