Can you swap out the optical drive for anything else?

I've got a mid 2012 13" macbook pro with the i7 processor. What I'm wondering is if I can swap the optical drive for anything other than another hard drive. I've already swapped the HDD for a SDD (WOW by the way) upgraded the ram to 16 gigs. I know that the optical swap is the next thing to do but I don't want another hard drive as what I've got is plenty. Any suggestions?

Thanks!