My approach (from a VFR only perspective) is closer to Peter's. To be any use, a realn alternative has to have differing weather. So major topographic features, sea, stuff like that make airfields worthy alternatives.
So called 'crash' alternatives - I've had to use them 3 or 4 times and at those times the 'nearest' button on the GPS, alllied to a flight guide are fine - the weather is still flyable, loads of fuel all sorts of things are possible.
Crossing the Channel on a poor VFR day - often the only viable alternative will be on the side you started from. Typically the majority of diversions I have made have come from reaching a point where I was not longer happy VFR and usually turning through an angle much greater than 90 degrees. Might waste some time but there is nothing much more comforting than retracing your path through a 'known' environment.