Places in Gloucester:
The Cathedral - there is a sculpture exhibition well worth seeing and it ends on 31 October. Crucible2 at Gloucester Cathedral

National Waterways Museum. Waterways Museum

Places in the Forest:
Speech House.
Clearwell Caves.
Puzzlewood.
Symonds Yat

This website may be helpful to you: http://www.wyedeantourism.co.uk

No doubt further suggestions will be forthcoming from others.

Mike