We are in our young 80's and after our RS BOE tour we want to go to Cambridge. We leave about 9:31 and get there about 10:30. Itinerary: Trinity College and Wren library, 2 hour walk with guide from TI, Fitzwilliam William, and ride on a punt. If I buy return tickets for 6 or 7 pm., can we come home on an earlier train if we get too tired? My husband has 2 knee replacements so he gets tired of museums and tours etc, I am perfectly capable of staying longer, but it would ease his mind if he knew he could come home early.
After 9:30, you can walk up and buy anytime return tickets. I'm not sure that's as cheap as advance, but it's not bad and provides maximum flexibility and I don't have the stress of a set time.
£38.10 anytime tickets return per person are valid on any train all day.
If you travel outbound after 9.30 from Kings Cross or St Pancras (earliest available is 9:31 from St Pancras or 9:42 from Kings Cross - they are stations across a street and effectively one large area) you can go for £26.20 per person. With that off-peak you can return on most trains at any time in the afternoon or evening, just walk up and get on with your off-peak return tickets.
If you were doing as much a £100 worth of total trains I would suggest saving a third with the Two-Together Railcard but if this all the trains you are riding it doesn't pay off - you have to pay £30 to buy it plus two photos.
It's a bit of a walk -- a mile or so? -- from the station into the center of Cambridge.
It is a bit of walk into town. There are frequent buses. Or you can visit the Botanical Garden just across the street and then in to town.
There is also a taxi stand right outside the Railway Station. There is another taxi stand on St. Andrews just beyond where it splits into Sydney and Hobson. That's a long block east of the Market Square. There are buses too, but I've not yet figured them out. I ought to, but since I'm usually there on business I default to the taxi. Panther and CamCab are the two I've used the most. Both have apps. :)