We took a 3 hour free walking tour that brought us all through Prague showing us both historical sites and current tourist attractions. We got to see the Charles Bridge (amazing views, you cannot miss it!), the Prague castle, and the Petrin tower (if you're afraid of heights, maybe take a hard pass on this).
Charles Bridge
10 Euro Boat Cruise
We also took a 10 euro boat booze cruise tour down the picturesque river at sunset, truly breathtaking! Prague is a great place to meander and get lost in the winding beautiful streets. That being said, any time after 10:00 the streets can get a little dangerous for young women so make sure you are very alert (pit pocketing, cat calling, etc.).