Certified 18 year old here.
-Band posters (Whoever you like but extra style points for Pink Floyd and classic punk show flyers.)
-Classic movie posters (Old godzillas, Fight Club, Dawn of the dead, etc.)
-If you're going for a more nerdy vibe then try some indie game posters and classic anime posters (Akira, Lethal League, etc.)
-Interesting art. Find some local artists or paint something yourself. I have a cool ass spray painting hung on my wall.
-Stringed instruments (Bass/guitar)
-Flags and banners
-Cork boards if you have things to stick
-White boards
I'd try to avoid anything too childish, but then again it's your life, do what you want.