Another easy one. Stan mentions wanting to get his toothbrush as early as Part I. The toothbrush can be found in his shack at the shipyard on Mêlée Island. Sadly, during Part I, the shack is boarded up and Guybrush can only peek inside and see a glimpse of the item in question. Fast-forward Part IV, when Guybrush is back to Mêlée Island, the same shack is half-destroyed by an earthquake and the toobrush can be looted. Stan is no longer on this island though, he was found guilty and sentence to Ice Quarry on Brrr Muda Island. You'll need to get there and to Stan following the main questline, so, when you finally do so, give the toobrush to him at last.