To find the Guest’s Keepsake Key in Dead Island 2, travel to the alley between Venture Walk and Stoker Pl in Ocean Avenue.
Once there, you’ll find the Serling Hotel Guest.
Defeat him then loot the Guest’s Keepsake Key! Now that you have the key, bring it to the Serling Hotel and unlock the Guest’s Keepsake Box in one of the rooms. As a reward, you’ll receive the Energizing blueprint.