What to do with old mini?

File server. The Mini is GREAT as a tiny little headless, low power, out of the way, forget it's even there, file server. It can also be your disaster recovery system. Should anything happen to the new Mini, the old one can take its place while it's in the shop.

It could even be a guest system if you have a guest room, while still functioning as a file server. Only problem is if it crashes or your guest shuts it down while sleeping, it'd be a little difficult to get in there to turn it back on or deal with it.

Those are just two possibilities.

Give it to someone in the family who could use it. Our down-level equipment has gone to my mother-in-law over the years. Maybe someone in your family really should convert to mac?

My old one is connected to a TV, primarily used to watch hulu and netflix.

Either sell it off, apple machines usually get good prices at the second hand market or one of the other options already mentioned.
From my last three mac minis, one was sold off, with 30% less than the original price, the second one went to my father, and the third one will stick at least 1-2 new revisions until Apple lowers the price again. (I see no reason for now to upgrade, I would get less for full price)

How do you do that?