I think that Cambridge would be a very good fit for the girls, a little more hassle for the arrival at the airport. Heathrow? Gatwick?
But once there there is an abundance of things to do for all ages. The colleges will have had their exams so some colleges could be visited, there is lots to see just walking around, the aforementioned market, and lots of interesting shops nearby, plenty of restaurants and fast restaurants, and food stands at the market, there is the Fitz - the Fitzwilliam Museum, world class and free, and across the street is Fitzbillies which make the most incredible buns - they now have a coffee shop, tea rooms and a sales counter. Seriously yummy. Then the nearby Imperial War Museum Duxford for lovers of flight and the US AF involvement in East Anglia, and the very emotive Cambridge American Cemetery and Memorial - a place I love. And punting on the river.
There are also loads of interesting things in the area - Ely, viking areas, Bury St Edmunds which is medieval, lots of activities around the Fens, the Cambridge University Botanical Gardens, and gardens at some of the colleges.
If you have particular questions ask away - I live just down the road.
By the way - I may have made it sound difficult to get to Cambridge. I prefer trains to coaches, and the trains are actually pretty straightforward: Heathrow to Elizabeth Line to Farringdon to (tube or train, one stop on either) Kings Cross station and the 52 minute fast train or stay on the Thameslink train (be sure it is the right one) all the way to Cambridge. Or for Gatwick grab a Thameslink heading north and follow the directions above. Don't do anything at Farringdon and depending on the destination of the train you boarded at Gatwick either get off at St Pancras if the train is heading to Bedford or stay on if it is going to Cambridge. If you get off, just cross the street and get a Great Northern train from Kings Cross (less than 5 minutes walk) to Cambridge, many being nonstop.