One thing I’ve learned from travelling a lot on a budget: small savings add up surprisingly fast.
1) Travel by regional trains and buses whenever possible, especially outside the big cities;
2) Eat where the locals eat. Lunch menus, bakeries and supermarkets can save you a lot compared with touristy restaurants. Also if you're going to unfamiliar cllimate, be care about strange local food to avoid avoid hospitalization;
3) Stay a little outside the centre, but close to good public transport. Often you get a better place for much less;
4) Check city passes and transport day tickets before buying individual tickets, sometimes they’re a great deal, sometimes they’re not, so always do the maths;
5) Get rid of roaming and use esim. Moreovere I recommend you to install it in advance. I used Kaspersky eSIM Store app to find propper data plan, it has worked well for me and it’s handy for maps and transport without unexpected roaming charges.
And my biggest budget tip: don’t try to save money on absolutely everything. Pick the 2-3 things you really care about and save on the rest. That way the trip still feels like a holiday, not a financial survival challenge!