Toddler group every morning except Thurs in the Catholic church hall in royston (nothing to do with the church, just uses the hall). Various toddler groups in the local primary schools just starting up, and also in Melbourn if that is closer.
Shepreth Wildlife Park does termtime season tickets; ds2 and I go about once a week for a quick look at the tigers followed by an hour in the playbarn
There's a tumbletots-type-thing just started in Royston - can't remember details but there was a poster up in the library!
Royston Leisure centre does parent&toddler swimming twice a week (the sort where you go in too, but they do actually teach swimming, or at least water confidence)
Leisure centre also does a bouncy-castle/ball-pit thing about 4 times a week.
There's a toddlers learn spanish group, I seem to remember.
Library singing and also story time sessions.
"Music with Mum" for less crowded music sessions
Parish church do "Pray and Play" toddler church one afternoon a fortnight.
Wimpole Farm is great especially if you're a National Trust member
There you go, music, gymnastics, linguistics, playgroup, animals.
If all else fails, we like to get the park'n'ride to Cambridge and go to the dinosaur museums.