Dartford is a useful place to leave from, which is easy to forget if you only use the station for work. The Southeastern lines run north to London and east toward the Medway towns and the coast, so a Friday evening train can put you in Whitstable, Margate or Broadstairs without a car and without a night in a London hotel first.
Pick one coast, not three
The mistake is to treat the Kent coast as a single destination. Whitstable, Herne Bay, Margate, Broadstairs and Deal are different weekends. Whitstable is the oyster town and the harbour walk. Margate is the Turner and the bay. Broadstairs is smaller and better if you want a beach and a town you can cross on foot. Choose one and stay there. The trains between them are fine for a day trip, and a poor way to spend a weekend moving luggage.
How to leave Dartford
Check the last train back before you book the room, not after dinner on Sunday. A weekend that depends on a rail replacement bus from the coast is a different trip. If you are going on a Friday, buy the ticket when you know the train, and keep the return open only if the fare really allows it. Dartford station is straightforward. The complicated part is the connection further east, so read the whole journey rather than the first departure time.
What is worth the time
On a two-night stay, one long walk beats a list of attractions. In Whitstable that is the sea wall and the harbour. In Margate it is the bay at low tide and the old town behind the front. In Broadstairs it is the cliff path toward Stone Bay if the weather is kind. Eat one meal you booked and one you found by walking. The coast is small enough that a reservation for every sitting makes the weekend feel like a schedule.
If the weather turns
Kent in October and November is not a beach holiday, and that is fine. Book a room you are happy to sit in, take a coat you can walk in, and keep one indoor plan: a gallery, a long lunch, a bookshop. The weekend still works when the wind is up, as long as you did not come only to lie on shingle.
Go for two nights, one town, and a train you have already checked in both directions. That is the whole trick of leaving Dartford for the coast.