Shelves are good! Bunny height ones
Then they can have the underneath space and the above space. It depends on your playhouse setup how you want it, but there are many ideas. Even a piece of furniture in there is a super easy way to get another level. Many people have enclosed areas with a door too, similar to a hutch? I haven't done that with mine though.
I've created some shelves using ikea lack tables and a piece of plywood&lino over the top. I've put stoppers on the walls to stop them moving. Nice easy way to do it
Make sure you have opening windows or a screen door, for ventilation. A plastic air vent from homebase worked well for me to get some extra ventilation.
Are you insulating? That's a definite good idea
Lino floor with a sealant around the bottom (to stop any wetness leaking through)?