This ^
I put the date back to January 2011, then get ten coins and change it to January 2nd, then 3rd...
I change the date back before I play any games though. Activity log would get messed up and say you played Ocarina Of Time (for example) way back then, haha.
So I only do it when I know my 3DS will be in sleep mode for a while.
edit: I just found out i can change the year backwards, exit to home, then change it back to the real date right after and the steps for the current day are reset! Nintendo needs to at least make the limit 20 or 25...