College Lakes Nature Reserve, near Tring, is good for a walk and to explore. It has a small cafe and gift shop and some great bits to visit around the lake with the kids. There is a 2 mile circular wildlife walk but if that is too far you can take the upper path to the right of the lake and walk for a little before you reach and outdoor (but under cover) old farming exhibition and a building which houses hands on wildlife bits.
There is also bird lookouts where you can watch the birds on the lake. When we visited there was building where you could watch nesting birds on a screen from a hidden camera on the site.
Parking is a recommended donation of £3 and its worth mentioning that scooters, bikes and dogs are not permitted. We have bought cream teas before on Groupon for there so check out Groupon before you go if you wanted to get a deal.
Access is mostly good for pushchairs.