Beng oblivious doesnt mean being dumb. Dumb girls are actually are turn off for most guys.
If your in a group sometimes act as if your not paying attention to the conversation and start to become obsessed with the back of your hand or your nails.
Or maybe tell him a joke that's a bit immature such as
Why didnt the skeleton cross the road? Then stare at him for a while and say cutely " ill tell u?- because he had no guts" then giggle in a cute sort of way. People would probably be laughing at your cuteness rather than your joke and even if he doesnt admit he will find it extremely cute.