A few of the roads suggested are a tad far from Monmouth, but you should try the wales weekend next year.
For local roads, if you take a drive west (above the newport, cardiff, bridgend area) there are some cracking roads. I will try and dig out some road names, but i grew up in Bridgend and heading north from there towards Hiraun and the Bwlch are some amazing roads. Myseld and Wizurd also found some great A roads as we headed back from North Wales this year - if memory serves they were coming down the Hereford roads.
Best bet is look on a map near where you are and pick some twisty looking roads