get new spark plugs and throttle body serviced along with Liqui Moly engine oil and get the original air filter cleaned if its in okay condition otherwise get a genuine one, do not compromise on maintenance parts.
besides, Prius has to be driven differently than other cars, it has specific driving methods. Search on Google about how to maximize the efficiency of Prius and visit PriusChat forums too.
when i floor it for a long while, I manage around 15 km/l and when I drive it peacefully, the average is around 18-20 km/l. AC is on all the time, even when the car is parked somewhere and people are inside.