I used to be on a forum with a very extensive currency system, and perhaps some of the details from that could be useful here, though there are definitely elements that are waaaaay too complicated and just wouldn't work here

- currency could be earned just by browsing the forum. You might get 2-3 coins per 10 forum pages you looked at (with a time limit so you couldn't just click a thousand threads at once and make money from it)
- there was a fighting minigame, where you'd earn currency by defeating enemies
- site owners often held mini forum spam games where people could win money (ex. a counting game where anyone who gets a multiple of 10 gets that amount of currency - it could get very competitive!)
- contests to introduce new avatar items into the game

The meat of the currency system were the avatars, which were essentially customisable paper dolls. Currency was spent on buying clothes, hairstyles, and companion pets for them. Something like this could potentially work on a much, much smaller scale for TSP.

Money could also be spent on:
- improving gear and weaponry for the fighting minigames
- functional things like new forum backgrounds, post backgrounds, new badges on your profile
- purchasing gifts for other users - for example, at Christmas there would be PM cards that you could buy and have sent with a small message, to the people of your choice
- tons of user based economy, with raffles, best-dressed competitions, 'clubs' with an entrance fee, charities for new users (who started with almost-naked avatars - the horror!), trading rare items for art... etc.

Hopefully this might spark some ideas. It was a fun forum economy to be part of.