The silvery animals dart across the room, weaving around and leaping over furniture with a grace that enchants the young boy sitting cross-legged on the floor. Awe illuminates his face as the doe streaks past him, and the stag stops to affectionately nuzzle at the air beside his neck. Patronuses aren't solid, but Lily can't imagine that tangibility could make Harry any more delighted than he is right now. There is something uniquely special about watching the ethereal creatures traverse the room, even for her.
The stag and doe slowly fade, but Lily and James raise their wands before the baby has the chance to start crying at the loss. She doesn't even have to bring a memory to mind; the scene in front of her is so perfect, so innocent and pure and happy, that it's enough on its own.
"Expecto Patronum," they intone, and the silver pair leaps forth again. The sadness that has just started to cross Harry's face gives way to joy at the sight of the otherworldly pair darting around him once more. And, leaning into her husband's embrace, Lily feels bliss.