Comment and I'll:

a) tell you how I know you and a thing or two about whatever you have in your gallery OR Tell you why I friended you
b) associate you with something - fandom, a song, a colour, a photo, etc...
c) tell you something I like about you
d) tell you a memory I have of you,
e) ask something I've always wanted to know about you,
f) tell you my favorite pic of yours
g) in return, you must post this in your journal