SOOKIE: She’s bringing baked goods to a boy. Wow, serious!
LORELAI: [in high voice] “Here Dean, these cookies are for you, ’cause you’re keen.”
Keen is outdated teenage slang meaning nice, great, cool, groovy, neat. Lorelai seems to enjoy teasing Rory by doing a mock-imitation of her as if she’s in the 1950s.