Girl Rats Having a Hard Life

Question
My biggest concern at this given point is their happiness.  When I bought
them, the girl at the Pet Store said they hadn't been handled much, and their
behavior shows it.  They are still very loving, don't bite, but one is aggressive
towards the other.  But they are trying to escape their cage.  

I need some ideas on what appropriate free-run areas could be created for
them.  I can't let them run free throughout the house, due to their destroying
clothing and the like.

Any suggestions that I can use and implement to make my rats happier.  
Their cage is currently a 3 foot tall bookshelf that we converted.  Maybe it's
not big enough?  I'm very concerned and sad.

Answer
That's big enough, but in the same instance that our homes/apartments/condos/etc. are big enough for us. Would you want to spend your whole life inside?

I like to rat-proof a bathroom or hallway (some people use a closet). Clean out everything valuable, all wires, and remove clothing you're concerned having eaten. Check walls, cracks, under sinks, etc. to be sure there are no rat-sized holes or compromised situations that may lure a rat. Set up a med-low, slick wall (I like the WonderWall) that your rats can't jump or climb, lay down a bunch of toys and some treats, and let them enjoy themselves for 45 minutes to an hour (or more if youve got the time). The best toy you can give a rat is YOU!! they love to explore and "bug" their people.

Good luck!