Stock or index fund parking place recommendation?

I have an online trading account that I use more for a hobby than anything else. On the rare occasion that I think a stock is going to jump, I'll get in, and sometimes get out pretty quickly... and sometimes just have to wait it out.

My question is in regards to the time when I'm not "in." I know zero about index funds, etc, but looking for something with only mild risk that I can easily get in and out of.

Suggestions?

Maybe even two "opposites" to offset each other's potential loss? Or is that just too darn conservative for a "hobby account"?