I think I have found my first improvement. It's a front radiator/grill cover. There are some stock covers for my car on the market, mostly for VAG cars (VW Skoda Audi Seat) Thanks Google!
It covers up the biggest opening and I wonder if it will reduce drag and will warm up the engine faster.
@ ptitviet: there are also grill covers for Octavia models:
Octavia III - winter grille cover in great OEM design - KI-R : superskoda.com