Hostels, pubs, trains, museums, public transit, walking down the street, tourism offices, accommodations booking offices - pretty well anywhere YOU go, as a backpacker, others will be in the same general area, so your chances of meeting other travelers are pretty good - just be open to talking to strangers and you'll soon find plenty of people to talk to and hang out with. Traveling in the off season there won't be as many others around, but I doubt you'll be the only person in any hostel, and if you are, chat with the staff and get all the inside info.