Go for a walk on Devil's Dyke? Have an amazing veggie meal at Terre a Terre? Have an amazing meaty meal at Dig in the Ribs (Hove)? Go up the BA tower? Sit and Totty watch having coffee in one of the many cafe's? Explore 'The Lanes' (overrated though imho), Royal Pavilion?
But Newhaven? IIRC it has an old fort other than that just somewhere to drive through on route to either Dieppe or Alfriston