You have not stated how many KM on ODO? Please re-confirm/check the following as some may not be needed:
1. If spark plug are Iridium tip then change on or near 100,000 km. But do see the condition and decide.
2. Clean the throttle body. This will be partial cleaning of critical items related to fuel economy.
3. Check and replace air filter if needed. Although air filter now in modern car has less to (not saying "nothing") do with economy; more related to acceleration response.
4. Clean the injectors. One easy way (if not too much of an issue with injectors) would be to use Toyota's original injector cleaner by putting one bottle in tank with 30-35 liter of fuel (follow the instructions printed on bottle).
5. If dual VVTi engine then may be not needed; if other engine then valve clearance checking is needed around 80,000 km. Apology as I do not know which engine in 1.6 Altis 2013. For example in current Gli 1.3 A/T or M/T it has 2NZ-FE which needs valve clearance adjustment check at 80,000 km but Altis 1.6 and 1.8 have 1ZR-FE and 2ZR-FE engines respectively which are dual VVT- and do not need such check. Please make sure this.
6. Clean MAF sensors if needed.
7. Most importantly do a OBDI scan i.e. connect diagnostic scanner and that will tell the performance level of all sensors. If sensors have an issue or near an issue you will know that. I would prefer to go to Toyota 3S dealerships for that (but still try to locate a learned technician within the 3S deanships; e.g I have located a fix person for my car within 3S dealerships)
8. Use good petrol.
9. Above all; if all the aforementioned items are okay. Then you have to reconsider your driving style as well. Try to adopt habits of Hyper Milling. Means to drive the car in such a way that you assume your car has no brakes and assume that there is an egg between your foot and accelerator paddle which should not break while pressing the accelerator.
10. I have never owned a 1.6 Altis and therefore can't tell you about its typical average expectations. My 11th Gen Gli A/T 1.3 gave me best economy of 19.5 km/l on motorway while driving at speed of around 85 km/hr :). My Grande CVTi 1.8 recently purcahsed is giving me 18.8 km/l at 100 KM/hr and 22.0 km/l at avg speed of 80 km/hr over a distance of around 50 km. Its hardly 1500 km driven and I will update on its typical average performance once I drive it more. Please keep in mind cold or less hot weather give you better fuel average than hot summers (in both conditions with A/C). My point here is that engine has better performance in colder ambient conditions than hot while all other factors being same.
11. Use correct engine oil specs. I think (plz reconfirm) that you might need for your car 10W-30 API SN; which is an oil on medium to lighter side. Do consult a tech while looking into KM on your car ODO and condition of engine.
This at least what came to my mind for now