I've not tried it myself but everyone here speaks highly of PAI.
http://www.projectai.com/ I suspect it covers selected airports depending on which options you choose to install. The alternative is to use TTools & AFCAD to add your own AI traffic. Detailed instructions are included with both utilities. Don't be too ambitious to start with.
Another method would be to install repainted textures to the existing Texture folders of the default aircraft. You could edit the Aircraft.cfg entries accordingly. This would change the liveries of the existing AI traffic.