I'd try a new gas cap, then a new charcoal canister.
+1. Just don't skimp on the new cap. Spend the extra couple bucks for a good one.
If it's not the gas cap, could be the VCPV (vapor canister purge valve). And IIRC from a few years back on my brothers '01 Camry, the purge valves on those are a PITA to replace, you need to drop the entire canister assembly to do it. Maybe others could shed some light on this?