I tihnk just encourage them to be creative and use their imaginations.
Encourage them to make things they want or need, rather than buy them. Browse pinterest with then, collect scraps of wood, cardboard, etc that can be used to make stuff, have a stock of other crafty materials (doesn't have to be expensive or 'sets')
Have a space in the house or their room where they can be crafty and spread out big projects to work on.
I build a very bid workstation across one wall of DS's room, using a big cheet of thick plyboard and an old shelf I cut in half to use as trestles that I mounted the plyboard to with big brackets. I then painted it in a high gloss so everything wipes off it and it's big enough for all sorts of projects.
Think outside the box.
Get them to make their own birthday cards for people, instead of buying them.
If you have a local bead shop get there and let them choose beads and supplies to make their own jewellery (very fun, creative and doesn't have to be expensive or use horrible plastic florescent beads).
Browse Pinterest for ideas.
Make papier mache models with them, at the moment DS and I are working on a huge papier mache landscape for himt o use when playing with his world peace keepers (mini soldiers) and toy vehicles.
Collect old cardboard tubes (especially big ones from wrapping paper or ask for some from fabric shops) and plastic bottles and make a marble run.
Let them use tools to help in their creativity. Clue guns, staple guns, drills, screwdrivers, hammers, hacksaws etc are all very useful when being creative. Childrne over 8 are capable of using these tools safely and properly with the right teaching and uspervision.
Let them use your sewing machine of you have one. Encourage them to turn their old clothes into clothes for their teddies or barbie or whatever. Or turn old clothes into bags or purses.
Collect junk and bits and pieces for them to use.
Be ceative yourself. Childrne like to copy their parents.
Don't alwaus make the outcome the important thing, sometime sthe process is more important. DS made a wooden bed for his teddy bear once. He spent a few days on it, learnt a lot obout cutting wood, drilling holes, and using dowels and wood glue to hold joints together. It didn't matter to him that is broke after a few weeks, the fun was in the making!
Hope that helps. There are lots of good blogs out there iwth ideas and advice and projects with tutorials, same on pinterest.