Is there any free way to have a Mac function as an AirPlay speaker?
Solution 1:
This isn't free, but it's only $2.99 AirServer
Solution 2:
Currently streaming from iTunes is only supported to Apple TV and Airport Express speakers. (Thanks @Jonathan)
You could try a pretty ugly hack of using home sharing to access your laptop music library from your Mac Mini and then control the playback by using vnc or some other remote desktop app to control playback on the Mini from the laptop.