Kidston Park is located between Helensburgh and Rhu, overlooking Rosneath Peninsula. As you look left, you can see the river Clyde and Greenock in the distance, on your right, you're looking at the Gare Loch.
The park has seen recent improvements with the construction of a new children's play area, additional new benches and a coffee shop. Unfortunately, the band stand didn't get approval for a renovation.
The park is very popular with families, especially on weekends and sunny days therefore great stealth is required. The cache is placed in a manner so that you can sort-of "hide" from the people enjoying the views but are too lazy to get out of their car. 
Bring your own pen.
Edit April 2022:
CSLT - Clyde Sea Lochs Trail
There is now a new panel at Kidston Park, very close to this cache. Take a moment to read it as it has some interesting facts on it.