I'd probably do a sitting area on one side with a low table and some chairs or couches, and might also doing some kind of mini runebook library type deal. Maybe not as necessary on siege what with only having one character so you usually just have all the runes you need on your person, but I find I really accumulate a lot of runes and end up with tons of books on me. Another option might be to show off some kind of collection. For example I have one of every kind of spellbook slayer type. So I could put them all on display with some benches like you would do runebooks.
You could do the backroom as storage, or more display with homemade pedestals and then collection type items on top, maybe statues or busts or something, they don't have to be rares to look nice. Throw some plants around, hang up some paintings or tapestries...maybe do a little shrine honoring your favorite virtue (or anti virtue).