The younger ones may want to make little fairy houses and then make a spot for them in the garden. They can put down stones and shells and moss and plant some flowers. You can make fairy houses in different ways but keep them natural of course.
Ol' fashion paper mache' (sp) is fun for the older ones, younger ones too it the project isn't too big. They can make landscapes, masks, planets, baskets like bowls, heads. Paint them of course
There are all kinds of painting techniques you can look into. They can do pictures with Q-tips and paint, dipping the ends in paint and form a picture with dots. This is just one of many
They can make paper dolls and clothes with paper and card stock. They can then make house from boxes or a castle with cardboard and cylinder type boxes for the corner towers - like from Pringles or oatmeal boxes. They can be cover with felt and glue or painted and fitted together. If they paint them be sure to put a coat of primer or white dull paint on them first.
They can do wood crafting of all sorts even small simple things like cutting a branch a few inches round and cut it in slices and sand and varnish and drill a hole for a tiny chain for making key rings, nice little gift.
They can make prints of leaves from trees onto a muslin banner size and tack it onto a tree branch and hang it with some jute attached or a knitted chain or something of the sort as a Summer banner/flag. You can do the same with flowers - deep shades of flowers and lay them down in a pattern you want, put a thin sheet of paper or cloth over them and pound them. Lift and see the pattern of the flower. These are nice.
You can make your own stamps with potatoes, cut different designs and use paint or ink to stamp onto paper or cloth. You can make a stamp by wrapping string around a block of wood any which way and painting it and then stamping onto paper or cloth.
Some kids love making books which they write and draw in and then sew together. Make a nice cover. Show them how the paper has to be put together as a small stack and then folded and sew down in the fold.
Well, these are some things off the top of my head, I'm sure you can find lots of things. Have Fun!