MTG

The first purchase I made was a commander deck. It's relatively inexpensive and has enough unique cards to get a good sense of what the different play styles are. We eventually mixed and matched to create a variety of decks out of those cards.

You could also buy a box of draft boosters. This is what we tend to do now when new sets are released. It's fun to open the packs and have a draft night before using the cards to make a bumch of new decks.