I just went fiddling around in my old school Mac mini the other day. I put some extra memory in, sneaking inside that tiny extra bit of space to pull out the old sticks and punch in the new ones. But the newer Mac mini is even smaller than mine, which makes it all the more impressive that iFixit has released a kit that will allow you to add your own second hard drive without a ton of trouble. Well, there will at least be a little bit -- Apple doesn't want just anyone rooting around in its tiny little desktop PC.
iFixit makes it about as easy as it's going to get if you don't want to pay someone to do it for you. There's a full guide here if you want to see how it all works. Though the kit is out of stock at the moment, when it's in stock it'll only be $70 plus the cost of whatever storage media you want to install.
I found dipping into my Mac mini pretty rewarding. As long as you follow instructions and are as careful as you can be with all of the components, you could have an even nicer mini than you've got right now.