One thing I highly recommend is visiting people. We had to pay for an airbnb twice, but aside from that stayed with Steemians or Couchsurfers (couchsurfing is free). This already saves a lot of cost, and if you're staying with friends, they might even include you in their daily cooking.

Aside from that, it helps to check how expensive different countries will be. While in Hungary, we re-stocked our "emergency food" (mainly cereal bars and dried fruit), because we knew that food prices there are ridiculously low (compared to most other countries).
If you google "food prices in country", you can usually get a breakdown somewhere of how high those are.

Before we entered Switzerland, we stocked up on cheap pasta and Pesto in Italy, to be able to cook. 6€ for 4 meals, basically.

In the end, it boils down to how much you can spend, you want to cook, if you want to eat out or see tourist attractions. We didn't enter many museums and mainly stuck to free activities.

Still, we love eating out, so most of our "free" budget (that wasn't tied up in transportation) was spent on food in restaurants.

Knowing how many calories you need can help ease your mind, we packed two bags of nuts, which would have provided enough kcal for a day - in case we would have run out of food for some reason.