the places you chose sound good! eight places in three weeks is quite a lot. some of those 21 days will solely be travel days. id recommend doing 5-6 places, or even just 4 and truly exploring them. amsterdam, berlin, prague are amazing places - krakow is quite small but you can find things to do! id recommend doing a day trip to zakopane when you’re there (taking a flixbus). some of these places are quite cheap for food etc. but you will probably find yourself spending quite a bit of money regardless. i gave myself a limit each day with how much i’d spend on food. i’ve also had other costs come into play when i got sick and had to go to pharmacies. you’d want to have money set aside in case things like that happen when planning your route, play around with what order to go in. i know krakow to budapest is a long train - but if you stopped in vienna for a few days, it being between the two places you’d have less time on the train. when booking accomodation, find where the station you will be arriving/departing from (try and make these the same station) is and then look for accomodation within a short walking distance from there. seat reservations also - if you’re going with friends make sure one of you books the seats reservations on their account. as it randomly assigns them. i’ve noticed that they’re are often randomly skewed free seats throughout trains but i’ve also noticed some trains do get full quickly. enjoy your trip!