PlusBus is a national scheme which allows to add bus travel for the day onto your rail ticket!
In Bristol its an extra £3.50 for an entire days unlimited travel with most of the city's bus operators.
Simply ask for PLUSBUS when buying your ticket at a staffed station. You can also add PLUSBUS to your ticket from most automated tickets machines and online rail ticket sellers.
More information: http://www.plusbus.info/bristol-plusbus