It's the school holidays and the weather is a bit pooky. Just because you're stuck inside doesn't mean the kids have to blob-out and do nothing. Using up energy is important. Some organising and planning will prevent boredom and frustrations. I have seen some good activities for preschoolers which don't cost much. Don't worry about the mess and ensure that helping to pack up is part of the activity. 

A few cool activities include:
Creating an obstacle course that traipses throughout most of the house. The kids can get to create the course and they can time each other or experiment running the course backwards or blindfolded.
Dancing is a great way to burn off some energy and musical games like statues or musical chairs are great. 
Making a hut. Pull out sheets and blankets and let them use the kitchen table, furniture or clothes horse as a base.
Useful websites for 5 to 15 year olds are:
www.learningmedia.co.nz 
and 
www.ncwilkinsons.com 
which has some multiple free games online.