Seaford Head is the essential walk from town: a chalk headland rising east of the beach, with big Channel views and the famous look toward the Seven Sisters. It is close enough for a short outing but dramatic enough to justify the trip to Seaford by itself.
The cliff edge is unfenced and the chalk can crumble, so stay well back even when everyone else is inching forward for photographs. Continue toward Cuckmere Haven if you want the full coastal walk.