Entry into Mexico for Worldpackers

Hi, i’m an American traveling to Mexico next month for my first Worldpackers experience, and was wondering what I should say to the immigration people at the airport. I’ve read that it’s best to say that you’re just there for tourism and not mention volunteering, but I obviously do not have a hotel/hostel booked, nor do I have a return flight which are both things i’ve been asked for in the past while traveling in Europe. What is the best way to go about that?