They look absolutely amazing when they're bigger! They remind me of morels / alien eggs. Unfortunately they grow painfully slow.

Lithops! They are simultaneously very hard and very easy to keep. You have to strictly forget them because one watering too much and they never forgive you. They're amazing especially if you don't have a lot of room, since they don't get big, although they need plenty of sun.

Or he's got a strange mood and wants giant platypus ribcage or something to craft a legendary toy train in which case he will absolutely get the wall treatment.