My idle coins currently do... Nothing really. There are no good investments for them right now, and they can't really develop any more value aside from sitting there and watching the value go up.
Maybe I'll start lending them, but there hasn't been any legitimate means of investing them yet. Maybe I'll find one soon.
use your idle coins in altcoin trading. buy etherium or Raiblocks and hold them for some time and sell them again when price increase. and you can use your idle coins in gambling also. if your luck helps you, you'll win more money from them. there are thousands of ways to use idle coins, just need to do a little research.