This isn't really a day out but I've just been to see groove on down the road on the southbank and it was amazing. Any child or adult would love it. Totally brilliant. You could have an amble around southbank eat them see that.
Tried geocaching? www.geocaching.com great for finding new places to go
Hendon RAF museum Lea Valley watersports centre Ashridge Estate NT park Bletchley Park (home of Enigma machine) Waddesdon Manor NT house and park Luton Inspire Swimming Centre (50m pool and diving boards) Knebworth Park Grand Union Canal walks or boat trips near Berkhamsted Thames river trips (commuter services, using Oyster card) Emirates air line cable car Thames Barrier - exhibition, walks, nature reserves Wetlands Centre near Barnes Kenwood House Museum of London at the London Wall Maritime Museum in Docklands Geffrye Museum Walking on the Thames 'beach' at low tide on the south bank