Via Beemama blog I found these wall stickers from E-glue. My son would be very impressed with one these giant firetrucks or tractors on his wall. E-glue from France offers a wide variety of decorations, individual ones or sets in different sizes and colors. There is even the possibility to have something custom-made.
E-glue is the most impressive wall decoration supplier I have seen sofar in terms of collection and color & size choices. They promise that their adhesive vinyl is sticky on all flat and clean surfaces, can be repositioned during the installation, and removed without leaving traces. A perfect solution for our rental house. You can order online and they ship worldwide.