You can buy a day return Preston - Bolton and a Bolton PlusBus ticket in one transaction at Preston station or on some online sites.
Preston - Bolton
Anytime Day Return £10.70 (Railcard £7.05)
Off-Peak Day Return £9.50 (Railcard £6.25) [Mon-Fri not before 0930]
Bolton PlusBus
A Bolton PLUSBUS ticket gives you unlimited bus travel on participating operators services, around the whole urban area of Bolton and the 'Greater Manchester' region - see
map.
Day tickets £3.50 (Railcard £2.30)
http://www.plusbus.info/bolton-plusbus-greater-manchester