Ok so this is quite an obvious one but often overlooked. Prices in most European destinations change drastically from season to season. The same meal that would cost you 15$ in July might only cost you 10$ in April. The price changes are significant in hostels/ hotels, restaurants, and adventure activities. June to September is peak travel season in Europe and best avoided. Winter, albeit snowy, is either completely off-season or peak-season depending on the destination. If you intend to see Europe on a budget, go during the shoulder seasons of March-May and October-November.