regarding Cambridge.
I'm not sure I can save you a whole heap of money. There is the national 2 for 1 programme from the rail companies but I don't see anything I would want to do in March. There are 6 results at https://www.daysoutguide.co.uk/search-results?q=cambridge but only 2 of those are actual 2-4-1 items, both at the same place. So 20% off a bike tour through a "partner", 10% off a punt company I have not heard of, 2 for 1 on what is called "group punting" (eh?? and in March with that temperature and those winds??) and 20% off what they call sip and paint?
That leaves two proper 2-4-1 experiences, both at the Museum of Cambridge. One an entry and one a tour. Got to say, I've been frequently in Cambridge for over 20 years and I have never been into the Museum of Cambridge, up on the Castle Mound. It just isn't convenient and why pay so much when the really fabulous museums like the Fitzwilliam and the Polar and many others are free? The Fitz really is worth it, and it has the added advantage of being nearly across the street from Fitzbillies, the home of the award winning Chelsea buns (worth every calorie, and there are a few) (take-out or eat in), and a seriously magnificent Chelsea Bun French Toast with maple syrup and crispy bacon (if you try to take it off the plate to take out it will go absolutely everywhere - eat in if you want to preserve any dignity).
If they had gone 2-4-1 on the red Citi-bus open bus tour that might have been worthwhile because in addition to the City it also goes out to the American Cemetery and Memorial of which I have spoken feelingly previously on the Forums. A very special place. But they don't (you can if you wish get a regular but less frequent regular bus there), and of course the Cemetery and Memorial are free. If you need or want more detail, ask.
Duxford is a wonderful WW-2 airfield and museum, part of the Imperial War Museum in London, and unfortunately not free, and not in town.
As far as getting to Cambridge from London, I suggest the train. There are three choices, train from Kings Cross station, or from St Pancras station, or from Liverpool Street station. All different train companies and speeds and prices. It may depend on where in London you will be as to convenience.
Greater Anglia run trains from Liverpool Street station. Cheapest, by a little, and slowest.
Liverpool Street station is on the Elizabeth Line, and the Central, Circle, Hammersmith & City and Metropolitan tube lines.
Thameslink run trains from St Pancras. A little less frequent and Kings Cross is just across the street.
St Pancras is part of a joint tube station with Kings Cross (Kings Cross St Pancras) which is on the Victoria, Northern (Bank Branch), Piccadilly, Metropolitan, Circle, Hammersmith & City tube lines.
Great Northern run trains from Kings Cross. The most frequent, and fastest, including 2 nonstop trains an hour which only take 48 minutes.
Kings Cross is part of the Kings Cross St Pancras tube station discussed just above.
As far as train tickets, Railcards only make sense if you are paying more than £100 on train tickets. You can save 34% on many tickets with a Two Together Railcard which costs £30 (goes up in Spring) but only after 9:30 am in the London area M-F. Railcards are not valid on Eurostar.
If your only trains are Paris and Cambridge I don't think they will pay off. You could avoid the morning rush hour ban by buying Senior Railcards for both of you but you would need one each which means you need to spend £60 which would need £200 of travel to start to pay off...
