Brighton and back is a well-worn favourite (plus many options to get a train back on the return leg if you've had enough).
I did a good ride from South London to Horsham and Littlehampton - certainly hilly and you get to finish at the seaside. The train takes longer back from there though.
On the Kent direction - what about Eynsford, West Malling, Cranbrook, Tenterden, Woodchurch, Sellindge, Canterbury - again plenty of trains back.