Music theory is the best and it only gets better with age! Here are some of the topics that I love!
The Circle of Fifths
The sweet Pentatonic set - Learn this patterns on you guitar to rock look the gods! (note: this patterns never plays the 4 or the 7 note in a seven note scale).
Diatonic Harmony helps point the direction of the chords so it easy to know what to play next!
Heres a short video its so easy!
On all my music theory sites I have a little music theory test. Try it!