I agree with David that the University Botanic Garden is well worth a visit (or more). It is only £4 per adult, free for children. There is a cafe and they have a picnic area. There is lots to see and they often have children's activities running.
Anglesey Abbey is only a few miles from Cambridge and is a famous National Trust House and Garden.
There may be some private gardens in the area open under the National Gardens Scheme (
www.ngs.org.uk). These will usually only open on one particular day. They only cost a few pounds entrance and usually have teas and plants for sale.
I do not know nurseries in the area but it will be worth looking out for a Plant Fair in the area as you would probably find several specialist nuseries there.
I hope you all enjoy your time here.