I'm doing some longer multi-day trips again this year and I figure perhaps I should add some training during the week with a loaded backpack rather than just going out once a month as I always do. Does anyone have any recommendations for something good to use in place of gear to get weight in your pack. I don't want to walk around with all my actual gear during the week. One blog recommended a bag of charcoal for the grill but I'm curious what others have used.