A few of my ideas:
1. Purchasing "moods". Using something like the XenMoods add-on except have some reserved for members to "purchase'. Currently, there is no way to do this as far as I know.
2. Purchase something collectable they can show as a page on their profile. Example, I run a football site and having cards of the various players would be outstanding. The ability to purchase, trade, win, etc. Such an add-on does not exist as far as I know.
Anyone find a use for their forum money?