Ely is another possibility, a fraction over the hour from London Kings Cross. It's a small city (about 20,000 population I believe) with a magnificent Cathedral. Very easy rail access to Cambridge, but also on the lines to Kings Lynn (for the North Norfolk Coast by bus), Norwich (all sorts of things to do in Norfolk), Bury St Edmunds and Ipswich (for the Suffolk Coast) and the Cross Country line to Nottingham via Stamford, Oakham and Melton Mowbray (all in Rutland/Leicestershire).
That also passes through Peterborough (for it's Cathedral)- actually also quite a 'green' city.
Even York is a doable day trip, at about 2 hours each way.
The multiplicity of rail routes means that the relative lack of useful bus routes from Ely nowadays doesn't much matter.
It probably isn't a particularly cheap place to stay.
Also Marlow and Henley are only short branch lines, not a significant inconvenience.
There are the good value Anglia Plus one and 3 days in 7 for all rail lines (except to Peterborough and beyond) radiating from Ely- https://www.greateranglia.co.uk/tickets-fares/discounts/rangers-rovers/anglia-plus