In Bangkok you probably want to look at Khao San Road, there's something for every budget and you'll also meet up with other travellers there. Personally I don't like KSR but it's very popular. You'll have no trouble locating a guesthouse as there are so many of them. Likewise in Chiang Mai's old city, within the city walls. I stayed there last about 2 years ago and there were so many guesthouses to choose from, and again to suit all budgets. We found a real nice place with AC, cable TV, good restaurants next door. Once you get into the old city walls it's very easy to find accommodation.