A combination of camping / hostelling / volunteering. We have good sleeping bags, a good 3-season tent and all the camping gear. We are used to camping - both of us are avid backpackers.
Since we are quitting our obs, we'll travel for as long as the money lasts and I want to stretch it as long as possible! Camping usually is the cheapest accommodation in Europe. I'll also look into Woofing (working for room and board in organic farms around the world) and Servas (you pay a small fee and you get to stay with families while traveling).