Walking And Cycling Along The Coast
The roughly 12 mile stretch of coast between Sandbanks and Christchurch form part of the National Cycle Route 2 and most of this is off road and level. This is pleasant route, whether walking or cycling, however it should be noted that some seafront sections are prohibited to cyclists during the peak periods of 10 am to 6 pm during July and August.
There are plenty of cycle hire places in the area, including one on the seafront between the piers.
Link below has excellent map showing cycle routes (off-road sections in green) and the red stars denote cycle hire locations.

Frontage
Bournemouth beach