point out buildings of scientific interest e.g. where scientists work (like a Doctors' surgery) or maybe even where a famous scientist once lived. A good example of a trail looking at buildings is the Bath Science Trail.
encourage people to look at particular plants they see on the trail.
have a trail about the geology of an area.
look at the different materials used in the buildings people will see on your trail.
help people to identify the wildlife they will see.