Poole makes a good base because so much of Dorset is within an hour.
The Purbecks
Corfe Castle is the obvious one: a ruined castle on a hill in a gap in the chalk ridge, with a steam railway attached. Studland adds beaches and dunes.
The coast
The Jurassic Coast runs west from Old Harry Rocks, a World Heritage Site for its geology. Boat trips from the Quay reach part of it without a coach at all.
Inland
Wimborne Minster is a market town with a notable church and a chained library, half an hour away. See Dorset and countryside and the New Forest.
Note the chain ferry has vehicle restrictions: check before routing a coach across it.