For trains it will work out much cheaper to get a return ticket and on the buses ask for "an all day ticket", that could save a lot. Buses may also have return tickets.
If using First Bus you will need to put the exact fare in the box when you enter the bus but drivers give change on some buses such as Arriva which goes to Paisley and Glasgow Airport.
Don`t be in too much a hurry that you miss out on this!
Also, there are special price excursion to places like New Lanark and Rothesay. These tickets can be bought at the main rail stations of Queen Street and Central.

