This is a truly great activity but you need lots of time and a few helpers don't go astray. It's perfect as a highlight in a school science festival or towards the end of term. Paint, foil or coloured paper are optional. Invent a challenge for the students - they must use the junk in teams to construct a device or devices to meet the challenge. Vary the difficulty of the challenge and the time allowed according to age and ability.
Older students can provide technical drawings with features labelled.
Invite older students to be on a panel of judges and give awards for innovation, practicality, economical use of resources, aesthetics and anything else you want to emphasise. Every class can contribute. Keep it in The Family Family Maths Nights are a regular feature of most school calendars now - and family science nights can work just as well. You can hold one big one or a few smaller ones during National Science Week. The Greatest Hits section here has loads of good activity ideas.