What media depends on what you are trying to acheive. You could fill it with LR rubble for added biological filtration, you could use a mechanical filter pad for filtering out large particulates along with a bag of carbon for chemical filtration. You could use macros like you mentioned if there is some kind of lighting over the filterbox, this would then help with nutrient export though I am not sure how effective the mini would be for this due to its small size.
Personally I would use it for a bag of carbon and some chemipure.