How exciting! Congrats!
I keep so much stuff around that I don't even think about it. It's too early to think this hard
Vet kit, thermometer with string, stethoscope, ointments, wound dressings & bandages, bute, banamine, etc...
Fence repair supplies (extra chains & snaps for gates too)
Extra buckets
Pitch forks, rakes, wheelbarrow
Heavy duty garden hose(s)
And always have lots of elbow grease handy
There's a lot more but hopefully someone else who's had their morning coffee will chime in!