I won't be back in time, due to attending Kim's Friday nightride from Birmingham.
However, you can't go wrong heading out on the little lanes towards Hitchin, as they're lovely ups and downs, as you'll be going over several ridges. If you go through Kimpton, watch out for the state of the High Street, as it's got more potholes than the Somme.
Emily's Tea Shop in Whitwell is highly recommended. Why not try a big loop around Stevenage heading out through Knebworth, Walkern, Weston, Hitchin, Lilley, Kings Walden, Whitwell and then back to Harpenden. About 50 miles total.