I travelled on this very route a couple of weeks ago and looked into the possibilities of doing a hauled train or two. It was not possible to get a price from Gdansk to Sopot, as the journey is too short. I'm pretty sure it was possible to buy tickets for the full journey, without breaking at Sopot, though. I can't check right now, as the reservation system is on its nightly 90 minute shutdown!
If you are prepared to travel on units, a very good value ticket is the Metropolitan Ticket. I successfully purchased one from the SKM counters in the tunnel at Gdansk Glowny station. There is a good chance of carrying out this transaction in English. The SKM ticket machines also have an English option, but don't seem to sell these tickets, so you'd need to be there when the counters are open.
Metropolitan Tickets cost 20 PLN for 24 hours or 40 PLN for 72 hours and are valid "in all buses, trolleybuses and trams as well as SKM trains and trains of Przewozy Regionalne Regio (PR) within the area of the Metropolitan Public Transport Association of Gdansk Bay (MZKZG)." (Quote taken from
http://www.mzkzg.org/?subpage=pod&art=35
SKM trains call at many stations, but are frequent (at least every 15 minutes, even on a Sunday after 11:00), whilst the PR trains are less frequent, but a fair bit quicker. The units are quite interesting to travel on - quite a throwback, although there are many different ages of interior.
It is worth noting that the Metropolitan Ticket is not currently valid on the new train service to the airport. However, tickets only cost 3.50 PLN single, so it's no real hardship.
Finally, if you do go for a Metropolitan Ticket, don't forget to validate it! There are validation machines in the tunnel at Gdansk Glowny.