Legolas himself is in the garden, packing snow into snowballs and finding a good place to hide. He doesn't expect to see another elf out here, because what else could she be if she's walking over the snow like that, with him. So when Tauriel approaches, he dives for his hiding spot with only a few of his snowballs and peeks out at her.

He tries hard not to laugh as he watches her, reaching for a snowball, before chucking it at her.