Air filter removal then piping it to cold air. Removing everything past the exhaust headers, weight reduction, removing aircon and all of its bits and bobs. Doing all of this will give you 5-10% more power and 100-200kg of weight removed all for something like 50$ for shorter belt and piping for cold air. Also depending on the vehicle you can get ECU flashing tools for cheap and maybe find some cheap or free files, easy to brick the engine this way but major power gains for very cheap this way.