How much would you pay your friend if they let you stay in their house?

She and hubby are both gone for weeks, so we are staying in their house while we wait for ours to become available (we're military, just moved) and we can get the mail, move her car, ect. Of course their super nice and expect nothing but I want to give them some money for their kindness. In this base housing you don't have regular utility bill. We're staying here for 2 weeks. What would you personally do?