The full range of local reserves of the Gwent Wildlife Trust is available here www.gwentwildlife.org/.../reserves Most are small reserves which can be explored in an hour or two at various times of the year, (of course ground conditions can be an issue at times)
The local Gwent Ornithological Society also produce a map of sites reserves and other s in the area here ... www.gwentbirds.org.uk/sites
There is a book on Birdwatching walks around gwent which may give ideas - currently not available from GOS but may be found at other stores if you can find it
http://www.gwentbirds.org.uk/publications?page=shop.product_details&flypage=flypage.tpl&product_id=42&category_id=1