Tidy up the local area, resources available from http://www.thebigtidyup.org/
Bin bags (for both landfill & recycling), tabards & promotional material - available for free from http://www.thebigtidyup.org/.
Litter pickers & gloves - provided by yourself (try schools, local council and other organisations).
Pick an area or two that needs clearing up, organise the scouts and set them loose!
Many councils operate a hot-line to report dangerous rubbish (e.g. needles or chemicals), take a note of this before you go out.
If you are using http://www.thebigtidyup.org/, register beforehand to get free resources & don't forget to log how much you collected!