While I’ve travelled my fair share, I haven’t made it to the whole world (yet), so this is based on my own experiences.
During peak summer, on little savings & no income, I survived 2.5 weeks in Southern Spain. Luckily accomodation was still very cheap as was most food and attractions. If you’re after a short break, there are many companies that offer cheap all inclusive deals to destinations by the beach. Or you could do what I did and backpack around by hopping on cheap bus fares between cities and living off tapas and cheap drinks.
Another cheap summer destination, it was not below 38 degrees for my entire 2.5 weeks so be prepared! Again, most hostels were reasonably priced in all of the cities, with Istanbul being slightly pricier. If you venture by yourself, rather than a tour, you can do all the attractions for a fare price as bus tickets are a steal. I lived off cheap kebabs for my entire stay and loved it all.
Whilst you do need to consider tour prices, tipping and be sure to haggle your way down for most purchases, Egypt is very affordable right now. Including entry fees (minimum 2 sites), a filling lunch and snacks, I was spending £10-20 GBP a day.
I booked Romania on a whim due to really cheap flights at short notice. For most of the year these prices are quite low, particularly between London & Bucharest. I splurged for a €50 day tour to Transylvania and Sinaia from Bucharest due to lack of time, but I found food and transport in the capital to be very reasonable.
The land of cheap beer & the beautiful old town square in Prague. Transport is reasonable using the teams and most food joints are reasonable, if you are willing to take away or sit down away from the major tourist attractions.
€1 shot bars anyone? Apart from the night I have little recollection of, I remember cheap, yummy, foods and everything in walking distance. Flights are generally quite reasonable to Krakow most times of the year.