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.
If you are using it for gambling then those are not your idle coins, atleast you are utilizing it somewhere, but do not spend on gambling its better to hold till the time of halving.