Casual:
* "Want some extra rice?"
* "There's some extra rice if you're still hungry."
* "I have some leftover rice, do you want it?"
More Formal:
* "Would you like some additional rice?"
* "There is some extra rice available, if you would like it."
* "I have a bit of rice leftover. Would you be interested in it?"
Humorous:
* "Anyone want to join me in a rice-eating contest?"
* "This rice is calling my name!"
* "I think I'm going to need a rice coma."
The best sentence to use will depend on the context and your relationship with the person you are speaking to.