I'm not understanding the layout of the room. Could you attach a few pictures or a floor plan?

For occasional adult guests (no more than several times a year), consider investing in inflatable mattresses. Then sparsely decorate the room so you would have room to inflate the mattresses as needed.

If your guests will mostly be children, a large thick gel mat on the floor and a sleeping bag could work for them.

You might consider built in cabinets on one wall within which are a pair of Murphy beds.

If the room has a vaulted ceiling or the height is sufficient (more than 8'), you might consider adding a loft.

Unless you actually don't like your overnight guests, you shouldn't even consider trundle beds or cheap "day beds" as the mattresses are usually really thin and very uncomfortable for anyone weighing over a toddler's weight.