Set up some props to center your activity, either outdoors or indoors. Mark this area by hanging some sheer fabric or paper streamers or use my idea - take a 10' by 10' outdoor tent canopy (borrow one if you can) and only raise it up to half-height. This makes a perfect castle area for pretend play, after you hang a few decorative flags and some streamers! You might want to designate one or two special chairs as thrones and have some space for craft assembly and "fine dining."
Of course, dressing up is always fun - see what you have in the way of royal garb and add fabric or towel cloaks, if needed. Jewels, such as party-store rings and beads of all colors go over well, also :)