Put the little pieces in a ziplock bag with gelatinous (sp?) liquid such as liquid soap. Seal the baggie and secure even more with duct tape or similar and then tape them to the patio door. This encourages the little ones to stand based on the height of these bags and they can try and move the pieces around inside the bag.
Save toilet roll inserts or wrapping paper cardboard tubes and tape them at an angle so they can shoot items through it. Balls and marbles are great if the children are older but dinky cars work well too. Just make sure they aren't the cheap-o dollar store ones which don't hold up so well.
Lay out a plastic table cloth on the lawn or any other easy clean up area and then mix shaving foam with food colours. Hint - strip them down to diapers first!