Have mini-encounters ready
Many games assume the GM will be providing most of the challenge and story. While this is a fun style of play, players are very unpredictable. Maybe it's thirty minutes into a four hour session and they've zipped through all of your prepared material. Maybe they ignored your carefully crafted plot and decided to go bar crawling. For these moments, having some random encounters can really help. Build a list of generic encounters which can be tossed in almost anywhere.
Mini encounters should range from five minute complications (reunite a lost kid with his parents, challenge someone to darts) through mini-adventures to tie up a few hours (A character is a witness to a crime and is asked to testify, a child is kidnapped, a raccoon steals supplies during the night). Fill out the collection with a few medium length complications (The local bully picks a fight, a mugger tries to mug a character, an accident sets a building on fire, a wild animal attacks).