Or just pick a song that you know the key of, and look at the fretboard, and figure out how to play it in your head without the lettered notes.
Lesson for today: the main riff of "Day Tripper" - The Beatles. It starts with an open E string. Take it from there. If you get stuck use the tabulature archive
here.
If you can imagine playing it, you can probably play it! I swear this works!