Mike noted " But, it was full of tourists " No it wasn't. Summer is when it is full of tourists. It's always busy because it is arguably the top touristic city in the world.
When you look at climate/weather data, keep in mind that temperatures underestimate how cold it is because it is a humid cold. As one of the world's top cities for walking the streets, dress warm & plan to be out of doors a lot.
You can easily spend 7 days in Prague, but if you want a day trip, go to Chesky Krumlov.
Except for the food, Christmas Markets are greatly overrated. The city is the star attraction.
Drink dark beer. I live in a Podunk town on the road between Denver & Salt Lake City, and we can buy Czech dark beer locally.