Haha, a marketplace was actually one of the things I was considering!
With both layout design to force a long walk, but also placing goods with synergies or sales. Maybe certain goods work better further along the park or just at the checkout, and some things like a sign or suggested recipe can make a customer walk backwards for a bit.
The endgame could be to make as much in sales as possible, with the classic "enemy health" being represented by a customers wallet / budget instead, and the goal being both to maximize total revenue as well as try to have no customers go through the store without buying anything.
It's probably the theme I like the most so far, other ones were traffic/shipping-based, but I like the sillyness of the store one :D
Though just reading through my description above I can already see the scope creep!