First off, this is a great dare. Most of these require some extreme limits. This one I think is perfect for adding a little excitement to your day. You can just roll and go do it!
Alright, let me get to my rolls:
What to wear:
6 - Nothing.
Might result in some crazy stuff. I'm excited!
Where to go:
5 - A window.
Probably the most dangerous outcome. Someone I know could be the one to catch me! 
What to do:
3 - Masturbate for 3 minutes.
Honestly, I'm glad I got this roll. I was gonna masturbate during the dare anyway, this just saves me having to do something else too. 
Extras:
2 - None.
A little diappointed I didn't get 1
or 3,
but I'll take it over rolling a 5
!
I picked my back window to do the dare; it's a pretty big window that goes all the way to the floor. Through the woods at 75 meters there's a dirt path where I see the occasional jogger. The trees do nothing to obscure the view.
I got to the window and took off my clothes. I was already very hard knowing what I was about to do. I wrapped my fingers around the base of my cock and started to stroke. I have to say the initial thrill almost made me cum right away, but that went away after the first minute.
By the time the 3 minutes were up, I was actually pretty used to being in the window. I could cum now, but didn't want to. I wanted that initial thrill back. I knew the exact thing to do it...
I went around to where the back door is. The view of it is even more exposed to the path behind the house. I opened it wide and stood right in it, which had the sun shining right on my naked body. The temperature and breeze were so perfect for the occasion! I continued masturbating and came pretty quickly, right on the back porch.
After cleaning myself up with a hand towel I got my clothes back on. Of course I'd already closed the door, as I have a bit of post-orgasm shyness.
This was a cool dare though, and I think I may revisit it sometime!