1.
Don't tell your dog you're planning anything: he'll feel sad momentarily but the happiness of the surprise will be worth it.
2.
Discreetly move the furniture so he's not ~too~ suspicious
3.
Bring a box in to the room, he will suspect it's for his birthday but he won't say anything.
4.
DUMP. THOSE. BALLS!!!!
5.
Let him bask int he glory of his new birthday ball pit:
6.
Allow for a brief existential crisis about how quickly time passes wash over him:
7.
But make sure it doesn't last too long: let him jump!
