Cats are adept at finding their own fun, often with household items or even crumpled pieces of paper that you inadvertently dropped nearby. But they seem to derive special pleasure from cat toys -- a variety of which are available at pet stores.
Catnip toys are good options, especially bubbles and teasers (usually wands with feathers dangling from a string), which involve both you and your cat. Marc also likes track toys, climbing trees, and tunnels; the latter two are particularly good for acrobatically inclined cats.
Before you buy toys, make sure they're durable, and watch out for small pieces that could be pulled off and swallowed. Look for canvas, heavy cotton, or hemp toys, which can withstand roughhousing, and be sure that catnip toys can be refilled.
Remember that you don't have to make all your cat's toys available at the same time; he or she will become bored less easily if you alternate. You can also make some simple toys at home, such as a paper grocery bag with its top folded down for a feline hiding place, the aforementioned crumpled paper, or even a plastic drinking straw. Just be sure keep any loose strings, elastic, ribbon, or plastic bags beyond your cat's reach, as they are all potentially harmful.