I want to play around with slo-mo videos but don't have proper gear for it. Is there a way to get my D5100 to shoot at a high fps (maybe by reducing the video res)?
I think you're out of luck.
Official specs say 30fps max, and I can't find any documentation of a possible hack.
The most widely distributed slow-mo camera is probably the iPhone 5s-- I bet you could borrow one to play around with.
The base limitation comes out to 30fps. MagicLantern (for Canon) and the Lumix GH2 hacks are two of the most advanced hacks out there right now, and neither of them has been able to get faster sampling reliably working beyond what is normally possible.
It really comes down to a basic hardware limitation in terms of how fast data can be moved and how fast the sensor can be read, cleared and reset for the next frame. If the sensor can't be cleared for the next shot more than 60 times a second, there is no way to get faster than that no matter how low bit rate you go.