Mastodawn

There are a bunch of penis themes to go for if you're looking for ideas:

#MonochromeMonday / #mcm: post your penis in black and white / monochrome to appreciate it in a different way.
#TummyTuesday: show off your belly and penis in all its glory. There's also #TesticleTuesday so you can show off how it's hanging.
#WankWednesday / #WednesdayWanking: these are often videos, but can be a series of action shots about how you pleasure your penis.
#FatCockFriday / #fcf: the name says it all, show off your fat cock.
#SoftCockSaturday / #scs: post your non-erect penis.
#SundayPenisPosting: end the week off with a great photo/video of your penis.
Bonus: if you're uncircumcised, there's also #ForeskinFriday, where you show off your hood in action.

And of course, you can describe your masturbatory adventures with #MyBateToday on any day.