So I could set up a commitment contract stating I must do each of these activities until my anxiety has decreased to half of its initial level by the end of a certain date.

I think it's much better to commit to do specific activities then to commit to decreasing your anxiety by half. You don't want to have to think to much about what do when you are faced with your exercises. You want to just do them.

I'm pretty confident that after doing stuff like that for say, a whole week, I would have enough social confidence for almost all normal purposes, and social confidence would no longer be a problem in my life.

I don't think that's true. If you don't continue to stretch yourself after a week is over you will lose quite a bit of the confidence that you gained during that time shortly afterwards.

At least that true based on my own experience of doing socially stressful thinks everyday for periods longer than a week. Sure it does raises your confidence to some extend but it's not like I can now easily do what it did there at the end of the challenge.

Of the top of my head, some ideas would be: going to a city far away from my home and walking up to people and pretending to be crazy (knocking on people's doors and asking "have you seen my pet fish?" until I get the door shut in my face), going to clubs and sitting in the middle of the dance floor, or anything else which would be very socially painful to do.

I don't think there a need to do stuff that's simply stupid. A good day 1 exercise is to ask 30 strangers for the time. Bonus points if you do it as fast as possible and really ask every group of strangers that you come about.

Day 2: Do the same thing while wearing a watch yourself.

Day 3: Do the same thing while pointing at the watch you are wearing.

There no need to bind yourself to achieve insanely high results in a week. Just set a lower goal for a week and set a higher goal after the week is over.