Walking, running, exercising and meditating are obvious techniques.
Being aware of triggers, such as hunger and tiredness that increase negative thoughts.
Walking in a book store may be a better option, if you are a person who has chemical sensitivities related to smells.
A positive book is anything you put together in advance to make you smile or just give you a good thought. Examples can be pictures of happy things or events, positive quotes, notes from yourself or someone else.
The goal is to help move forward from a negative thought.