Even the totally uninspired will probably find something to interest them at the new Enjoy England site (visitengland.com), which is a vast improvement on its predecessor. Big-name attractions such as Alton Towers are listed at visitbritain.com/world/attractions.htm. There's a national museum portal at 24hourmuseum.org, clickable maps showing English Heritage properties at english-heritage.org.uk, and an exhaustive list of events at whatsonwhen.com/pages/family.jml. Nearby foliage can be located at the Woodland Trust (woodland-trust.org.uk/woods) and Forestry Commission (forestry.gov.uk) sites.
London
Much of the Evening Standard's website is subscription only. The Days Out guide at thisislondon.co.uk/entertainment/daysout - happily - is not, so you can still search for children's events, Roman forts, castles and towers, amusement parks and zoos. You could also consult the London tourist board's list of free events and exhibitions (londontouristboard.com).
North
Try York (thisisyork.co.uk/york/tourism) and Bradford's guide to local walks (thisisbradford.co.uk/bradford__district/leisure/walks). See also Leeds Today (leedstoday.net/LeedsLeisure/NewsFrontmore.aspx?sectionid=167) and Manchester Online (manchesteronline.co.uk/entertainment/daysout/).
Wales
Visitwales.com is the official site; Castlewales.com is a quirky, exhaustive collection of Welsh castles that will stimulate children's interest.
South
Should you be feeling hardy, the Good Beach Guide (goodbeachguide.co.uk) lists clean and well-maintained beaches. The Brighton and Hove Parents Guide (thisisbrightonandhove.co.uk/brighton__hove/leisure/parents_guide) is invaluable, and the Gratis Tourist Guide to Devon (gratis-devon.co.uk/links/devon-attractions.shtml) has a section on rainy-day activities.
Further away
The Parc Asterix adventure park (parcasterix.com/som.htm) may seem a long way to go for just a day trip, but Roissy-Charles de Gaulle airport is only 10 minutes away from the parc, and the site does its best to win over children who think they would rather go to Disneyland Paris (disneylandparis.com) by Eurostar. Legoland Denmark (lego.com) is bigger than its Windsor offshoot, and is attempting to build the world's tallest Lego tower. Maersk Air (maersk-air.com) flies to Billund airport from London.