Inside the Prag castle complex.. My family and I did a tour inside and it was a lot of fun. We learned a lot about Prague's history and its secrets that you wouldn't have directly known by walking on your own. Of course you can always check facts on wikipedia but I find it easier to remember when someone shares them with you. Especially if the guide is a local and is just as enthusiastic about the place as you are🙃. It was my second time doing a tour in Prague and i didn't get disappointed. You can feel how passionate and patriotic the guides or generally the people are about their country.
You can easily book any tour suited for you (bike,segway,walking,bus) online. The app I mostly use is called Get Your Guide. It offers activities for up to 7000 destinations around the world. The tour we did was Best of Prague by Foot and Bus and 1 hour Cruise. It cost 30 euros per person.. not cheap but so worth it.