One of the things my son loves to do in the store is go to the sport section and pick up the weights. He loves feeling the weight and pushing them up with his arm over his head one handed. We try several weights when we are in the store and he's always facsinated by how strong he is. Of course having a SPD child I can definately see the value in allowing him to do this or even holding one throughout the store until I get to check out at least provided he's not having a meltdown or in a mood where I wonder if he's going to hit someone with it.
There are several different weighted objects you can get for kids with SPD from weighted blankets, vests, animals, and so forth. You can end up spending a lot of money on these types of products if they're commercially made. Buying hand weights is pretty cheap. I like the kind that wraps around your wrists or ankles. I found a pair that did both on clearance. They were 1lb each and while it doesn't seem like a lot if you wear them long enough (espeically for a 5yr old) you can tell you have weights on).
However seeing this blog is about finding things that are cheap and are from home I do have some suggestions. You can use pringles cans filled with beans or rice and then taped shut. You can use regular soup cans that aren't open yet but if they're likely to fall you might want to make sure it doesn't pop the seal. You can fill some socks with beans and tie them shut. You can lift a backpack or a bag full of something in front of you. Carrying the laundry basket full of their toys or clothes is another one. (I'm all for having them pick up their own mess.) You can in the summer time play with water balloons but be careful if they have a sensitivity to having wet clothes. Have them push a wheelbarrow around and to add more weight you can add dirt, rocks, and maybe a sibling if they're big enough. These are just some ideas where they have to use their muscles to get that input they need and crave.