Another idea would be rolling, random periods where all users can use premium features.
I've seen that on free internet services, for instance. Every now & then when you log in, it tells you for a limited amount of time, you can access all premium features.
If Matt felt like it, every now & then, for the heck of it, he could toss a Happy Hour into the middle of the trading day on Tuesday. If it shakes some subscriptions into the hopper, great.
Sort of like when you suddenly get a stretch of free movies on Showtime or HBO.