Newmillerdam
Newmillerdam is a country park off the A61 between Darton and Wakefield. There are three lakes with sometimes muddy, but fairly flat walking around them. The lakes are in a much larger forested area where you can wander about and get yourself pleasantly lost, alone but not that far from civilisation. Many of the paths and tracks are very muddy so you'll need walking boots or wellies, and I would recommend an Ordnance Survey Map of the area.
Believe it or not these are actually colour photographs. From the lack of colour in them you would be dead right in thinking that it was a grey, grey day when I took them.
The lakes at Newmillerdam support a wide variety of ducks. On a pleasant day it is pleasure to sit and bird watch. Other than the walking and the bird watching there isn't a lot to do here. There are a couple of pubs and a cyclists' cafe in the village where you can get a decent meal, but not if you've a dog with you unless you leave him in the car. There are parking charges in the country park car park so many people park in the pub car parks and I suppose a good many of them actually eat there too.
You could add a trip to Nostell Priory or Pontefract and make a day of it.