upgrading your vehicle (or buying a new one), or on gifts for the girls, which improves your relationships with them.
When you're not competing, you can go out on dates with the girls you've 'unlocked'. You can decide where to go, including the movies, the aquarium, the beach, a luxury hotel, or a sports event (among many others). During these dates you have to answer some of the girls' questions.
Drift Girls is a very odd racing game, which is paradoxically a lot more entertaining when you're not driving a car. In a few words, the dating simulator part is much better than the racing simulator. Finally, it's impossible not to mention the exaggerated anime-style graphics ... which lots of people will definitely love.