My DCs' latest was a shoebox aquarium, which they coloured blue on the inside and stuck fish on (you can suspend them from string too)
One fun way of using up youngsters' paintings - you know, the ones before they actually start painting objects or people, and they just end up covering the whole paper! - is to make them into something new. When DD brought home all her nursery splodges creations, I cut them out and arranged them into a snowman shape complete with scarf - I then used bits of craft foam to make stick arms and a carrot nose, and buttons for eyes. He's been up in the kitchen for years now, he's like a part of the family! And he's much more special as he was 'painted' by DD rather than using coloured paper.
This half term I'll be getting some more craft foam and using them to make prints. I did this at an art class I went to recently - draw a simple picture on foam, cut out bits of it and stick it onto thick card, and use paint to stamp it. If you have pieces of card exactly the same size you can do two colour prints by putting some foam on each piece, making sure you line them up correctly.
We might also do a collograph which (if I understood the teacher correctly...) is sticking loads of collage stuff on card and then using that to make a print.
I've really gone off the crafty make-your-own kits lately, so at Xmas I'm probably going to put together a big craft tower with 'raw materials' like loads of tissue paper, lolly sticks, buttons, ribbons, fabric scraps from the cloth shop (DD loves working with fabric), paint, glue, beads, glitter etc so they can just come up with anything they think of.
Mind you at Xmas I will make an exception for Hama beads - nice peaceful activity - and planning to introduce them to Shrinkles as well (toy of my youth :o)
Not sure I could choose a favourite piece, DD has done some really nice drawings in the past. She draws or writes in bed most evenings.
We usually do painting outside, they also love using pavement chalk. Our pathways are currently full of planes, storms and shipwrecks, and an enormous house which they did with the neighbours.