Shay wants to take Tally out for a night hoverboard ride. But instead of breaking into New Pretty Town, or riding out to the suburbs where the parents live—because who would want to do that?—Shay wants to ride out into the wilderness, to go see the river rapids and the ruins of a city. (Whee, city ruins.)
You know, one last adventure before they turn her pretty and boring.
Tally argues that they can still do things like that when they're pretty, but Shay points out that pretties never do fun things like risking their lives on night rides.
So Tally agrees, in part because Shay is her new best friend.