One of Shawn's most overlooked displays of intelligence isn't related to deduction/observation which are Shawns strongest skills. In Season 5, Episode 5, "Shawn and Gus in Drag (Racing)," when he modifies Henry's old pickup truck for a street race.
I think this is particularly impressive because Shawn managed to install a functioning nitrous oxide system despite seemingly having little to no auto-mechanical knowledge. When Gus starts questioning whether he made the necessary modifications, Shawn doesn't even know what an intake is. Yet despite this, he successfully installs the system, gets it working, and nearly beats an experienced street racer driving a significantly more powerful, modified vehicle. Obviously, Shawn accidentally cutting the brake line demonstrates that his work was far from perfect, but I don't think that takes away from how impressive the accomplishment actually is.
Installing a functioning nitrous system isn't necessarily a simple task, especially for someone with virtually no experience working on engines. It requires at least some practical understanding of how different components interact, and Shawn somehow figures out enough to make it work. I think this is particularly interesting because it highlights a side of Shawn's intelligence that rarely gets discussed: his ability to improvise, adapt, and tackle unfamiliar technical problems. Shawn is obviously an extraordinary detective, but I love moments like this because they suggest his intelligence extends beyond observation, memory, and deduction. He's capable of applying his abilities to completely different situations, even when he lacks the relevant background knowledge.