I'll kick off with Van Hage's garden centre in Ware, Herts. Lovely Christmas decs. and lighting dept. Santa (free but they ask for a charity donation). Animal section outside. Cafe, plenty of parking just of the A10 (take the Ware turnoff but not the A602 one).
Springtime Nursery in Crews Hill (N. Enfield) is great fun. They have a Christmas ride (animated animals) it's lovely but not cheap at £5 per child. They have a large toy dept. (but what I'd describe as market toys not many labels). Lots of decorations, great fun but parking is limited, there are about 8 other nurseries to visit on the same street too.