Good things to keep on hand for minor bowel upsets and bulking of stool:
Plain canned pumpkin: Pumpkin contains a lot of pectin and fiber. It will help with loose stool OR constipation. Most dogs like it. Can't really overdose it. I split a can between my 3 on a regular basis.
Slippery Elm Powder: Slippery elm has been used by Native peoples and as a folk remedy for intestinal upsets for centuries. It has an extremely high mucilage content, and if very soothing to the entire intestinal tract. It has a nice nutty sweet smell, and most dogs (and people) like it. It is used sometimes as a flour base, you can eat as much as you like and enjoy it. It cannot be overdosed. Find it at your health food store. I usually put about a tablespoon in the food of a dog that is having issues.
Most any vegetable material is a good stool bulker, and/or will help constipation in dogs. The pumpkin and the slippery elm is a good combination tho, and will have most pups back on the "regular" road fast.