From Bulbapedia, the community-driven Pokémon encyclopedia.
The Suite Key is a key item in Pokémon Diamond, Pearl, and Platinum. It belongs to the hotel at Valor Lakefront but one of the visitors has lost hers.
The key can be found by using the Dowsing Machine in the area near the southern exit of the Valor Lakefront. Upon talking to the girl, she will take it back and enter her suite.
As a reward, the girl will give a Lava Cookie. That is the only way to get a Lava Cookie in Diamond and Pearl without using Pal Park or cheating. In Pokémon Platinum, she will give a White Flute instead due to the cookie being available in the Veilstone Department Store. She mentions that both of these items can only be found in Hoenn.