Audaxes
Audaxes are non-competitive events that follow a pre-defined route which include a number of checkpoints or "controls". These controls are usually at a cafe or something similar where you can rest and eat. Unlike sportives, audaxes are not waymarked. Each entrant is given a detailed description of the route, and is then left to find their way unassisted.
A rider can set their own pace, provided it is between the minimum and maximum specified for the event. Times for the route are not recorded. Events are on public roads, but routes tend to bike-friendly country lanes with little traffic. However you may have to be wary of road debris and cattle grids!
More details can be found on the Audax UK website.