after lightning POS integration, you can provide them with some nice stickers: one big for their storefront, small ones for customers visiting.
While onboarding, you can make use of competition between merchants. "Look merchant A is already using this, you'd better do it as well".
after onboarding a decent amount of merchants, you can visit city officials with some data from a swiss Bitcoin town and El Salvador to show the boost in tourism. They may support you perhaps with some activities or billboards etc.