This weekend is the Bristol Festival of Nature, the UK's largest FREE natural history event. 

It takes place across the city this entire weekend and there will be a multitude of stalls and events happening. I intend to get down there soon and will update you on how I thought the event was run.
According to the organisers over 150 organisations have a presence at the festival and there are 'hand on experiences, live entertainment, a local produce market and other interactive activities'.

For more details visit: http://www.bnhc.org.uk/home/festival.html