How do I keep me and my player happy in a one-on-one game?

As a GM, my comfort zone is a group with about 3-5 players. I like the diversity of having multiple players. When I only have a single player, I feel I can already predict my player’s actions, and we are not into the same things, and this can cause a problem. So how do I keep us both happy when we are doing one-on-one games?