-Air Filter
-Spark Plugs
-Throttle Body Cleanliness
-Final Drive ratio (check your glove box for a code on the sticker, GU6 would have best mileage, GT4 is midgrade and GT5 would be worst)
-Fluid condition. It appears you've already done this part.
-Low ECT, or a constantly-rich condition. Check with a live data scan tool.
-O2 sensor (upstream)
-Slight intake leak (enough to cause a call for more fuel, but not enough to make the system mad and throw a code)
-Tire pressure and tire condition. Follow the door sticker for pressure recommendations.
-Failed fan clutch
-CPAS wearing out or getting dirty (works well enough not to throw code, but is just riding the edges of the PCM's tolerance for actual vs. desired position)
-Top Tier Gas. These engines seem prone to carbon buildup with no-name gasoline brands. Top Tier generally helps to minimize further buildup and can sometimes help to alleviate current buildups over time, and may provide a little better mileage. Consider a bottle of Techron if you've been running non-TT.
-Exhaust manifold leak. Pretty common to get cracked, a small crack may mess with the O2 readings a little without actually throwing any sort of code.
-Partially-restricted exhaust. Maybe not enough to make the PCM mad yet, but could be an issue which is coming up. This can happen if your engine is constantly running rich, or if you have leaky injectors (which would also cause excess fuel use)
Really, there's just tons of stuff to check, and we can start narrowing a ton of it down with a live data scan tool.