We live in Bricket Wood and do a lot of long walks. The river path along the Ver in BW is lovely. I haven't been to Aldenham country park yet but plan to go sometime as others say it's great. Last week we had a lovely meet-up at Willows Farm (London Colney) although the entrance fees are a little steep but once you go in you can bring your own food etc and everything else is free I think.
There's a good park in Tippendell Lane near me - Greenwood Park is the name I think, has lots of play equipment.
One of our favourite things at this time of year is to go fruit-picking en famille, take all your Tupperware and get blackberries, sloes or even rose hips and make jam, home made wine and sloe gin.
Woodside Leisure park has a cinema, bowling alley and kids play centre. Also there are sports centres with inflatables in the swimming pools, trampolining lessons and lots of other activities for all the family.
There is a lovely walk from Garston to Bricket wood through the woods where you can see red deer. If you like pubs the ones round here all have gardens for the children and it's lovely on a sunny day. Moor Mill (Smug Oak Lane, BW) is particularly pleasant with the river running through it, swans, ducks, geese and fish for the children to feed, and a fenced playground area.
If you want to meet a few people why not come to our next meet-up. We don't have one organised at the moment as we only met last week but we will organise one soon, I'm sure. Look out for a thread entitled something like "Watford/Barnet/Enfield mums..."
Picnics are always great for keeping the cost down. We had a lovely one a couple of weeks ago down by the Ver in Drop Lane, BW. The children brought their fishing nets and spent hours paddling in the river which is small and manageable at that point (at the stepping stones) It didn't cost us a penny.