When you reach the Valor Lakefront, you will come across a hotel. If you talk to the NPC who is in front of the first building you see (the northernmost building), he will complain about how he lost the key to his suite. Now she can't enter his room. Since she won't look for her, she's up to you. This is where you can find the suite key in Pokémon Brilliant Diamond and Brilliant Pearl.
The NPC will give you a hint as to where the Suite Key is. She is sure she had it when she left the reception counter. While you may be tempted to check out the entire reception building, the key is actually in front of it. Run south to the reception building but don't enter. When you reach the building, look left at the small hill. Press Button A at various points on the slope until you grab the key. The spot is very specific, so you may not have it right away.
Once you have the key, return it to the NPC. He will thank you before entering his room. Follow her inside her and talk to her again to get a lava cookie. This cookie can be used on a Pokémon to heal the status conditions affecting it.