For me, I think it has to be going solo (eeek). I started flying nearly a year ago but due to unforseen circumstances, had to stop for 4 months. Just restarted end of nov, and have done an additional few hours in circuit since then. Freely admit the idea of solo freaks me at the same time as being a draw.
Once I've done that, I suppose the next goal would be to get my license, although I did tell my instructor my true aim was to be his longest standing student. He has now renamed me "pension fund"!
Oh yes, and one day to try a gyros