Household Items
Make a list of common household items and require the participants to pick these up. You can restrict the rules so that the participants can only get used items from generous people or from their own houses. Include items such as toilet plungers and potato mashers. Don't include items that are too expensive as people are less likely to donate or buy these items to complete the scavenger hunt.
Outdoor Items
Create a scavenger hunt that is complete only after the groups collect a variety of items that can be found outdoors. List common things like leaves and colored rocks. Other ideas are twigs or flowers. Include cigarette butts and discarded water bottles -- you may wish to supply rubber gloves to the group for such items -- so you are cleaning up the area and completing the scavenger hunt at the same time.
Food
Require as part of your scavenger hunt that the groups to collect food items. The items can be from a list that can make a recipe, or a list of food items that are common in most households such as sugar and milk. Alternately, include a list of non-perishable items that can later be donated to local food banks. Require the scavenger hunt participants to pick up no more than one item per stop in order to add to the number of stops that must be made, and to force choices to be made at each one ("We could get canned chick peas or macaroni here... which of those will the next store more likely have, so which is better to get now?")
Locations
Host a location scavenger hunt, or mix pictures of specific locations in among things the groups must get to finish. Provide the participants with one digital camera per group, or ensure at least one person in the group has a camera on her cell phone. Provide a list of locations that the group most take photos of in order to complete the scavenger hunt. To make it more of a challenge, require that all group members be present in each photo.