How safe is a light aircraft?
This is a very difficult question to answer. The problem with aircraft accidents is that they tend to be either very minor or fatal, with very little in between.
The other activities mentioned often involve very serious, but not fatal, injuries. I used to fly an Air Ambulance, and as such got involved with a lot of accidents involving serious injuries. A rough breakdown of those accidents is as follows:
Daily, approx' 5 accidents or medical emergencies (roughly 60/40).
Car accidents with serious injuries, 1 or 2 a day, fatal 1 or 2 every 10 days.
Motorcycling accidents, serious injury, 1 or 2 a week, fatals quite rarely.
Industrial accidents, about the same rate as motorcyclists.
Horseriding, serious injury, about 1 every 10 days.
Military training accidents, about 1 a fortnight.
Other sports, never.
Aircraft/microlights, never.