Compton Acres consists of a series of different gardens built on the side of a hill overlooking Poole Harbour. It's deservedly popular, so avoid peak times. Arguably it is at its best when the rhododendrons are out.
There are nice views over the harbour from the terrace outside the Bayview Restaurant, but otherwise the gardens feel very inward looking and are surprisingly enclosed for a hillside site.
Check the Compton Acres' website for opening times etc.