Mobile Phone plans for multiple European countries?

Hello all! My friend and I are planning on traveling to a few European countries in December & January for about 3 weeks, and we’re in the early stages of choosing which countries, how long we want to be there, what we’re going to do, and all the logistics that go along with that. One question of ours is regarding mobile phone plans! I know American providers charge an arm and a leg DAILY while abroad, and I’ve been told before to use local plans. We have unlocked phones, but since we may only be in each country for less than a week, what would the best course of action be? We’ll definitely want access to data when possible, and I’m hoping for the most affordable but practical plan possible. Does anyone have experience with short term phone plans in multiple European countries, or other recommendations therein? Thank you in advance!