Retford is a lovely town but pretty dead on a Sunday.
If the weather is nice King's Park is lovely, bug play area, tennis courts you can hire, in November the cafe may not be open though. There is a play centre called Captain Jack's near Asda. Bit small but might be good, would book in advance though as cam get busy.
There is a museum we have been meaning to go to but not sure it would be open on a Sunday.
As others have suggested, Rufford Country Park & Clumber Parl are both 15 mins away and nice to walk round.
Sherwood Forrest is nice for a walk round d, that is at Edwinstowe which again is about 15-20 mins away
Sherwood Pines again about 20 mins away, has Go Ape, various biking/hiking trails
Your issue will be weather,there is sod all to do if it rains.
Doncaster Dome is ok, has ice rink, pool with slides etc. It is very dated and I believe swimming is still restricted to a 1 or 2 hour slot following covid.
Lakeside outleisat Doncaster is nice, there is Moat Park behind it that is good for a walk round the lake.