Dogs, especially young dogs, like to chew things. Hopefully you have provided a wide selection and ample supply of chew toys for your foster pup to choose from, so that she won't be tempted to gnaw on your table legs instead. Manage her environment to remove temptations: if you leave your favorite shoes lying around in easy reach, and the foster pup destroys them, that's your fault, not hers.
If you're having problems with digging, the fix is even simpler: don't leave your foster dog alone in your yard. Some breeds, particularly terriers, dachshunds, and other "earth dogs," have been selectively bred for hundreds of years to pursue small prey underground. You can't expect these dogs not to dig; the instinct is in their blood. If the foster pup were your dog, and you had the space to spare, it might even be worthwhile to build a dedicated doggy sandbox in your yard. But since this dog is likely to be a temporary tenant in your life, unless you plan to dedicate yourself to similar breeds in the future, it's probably best just to manage the problem by limiting the dog's access to your yard and garden.
When you leave the home or are otherwise unable to supervise your foster pup, crate her. Put a stuffed Kong and a favorite chew toy in there to keep her occupied, and don't leave her crated more than a few hours per day, but use the crate.
Make sure your foster dog is getting plenty of exercise, both physical and mental. A dog who has acceptable outlets for her energy is less likely to burn it off by destroying your things. Puppies play constantly; healthy adolescents and young adults of most breeds need a minimum of 60 to 90 minutes of vigorous exercise per day. Mature adults still need 30 to 60 minutes. Walking on leash does not count as vigorous exercise for most dogs. Jogging, off-leash hiking, or romping at the dog park (assuming your foster pup is up to date on vaccinations and appropriate flea, tick, and worm preventatives, and is dog-social enough to enjoy the park safely) are better alternatives.
After the first week or so, walk in new neighborhoods: seeing new sights and smelling new smells is more interesting, and thus more mentally tiring, than the same-old same-old of your own block. Training is great stimulation; practicing Obedience 101 and schooling your foster dog in a few tricks are valuable exercises not only for their own sake, but in keeping your dog entertained and mentally limber.
If your foster dog only destroys things when you aren't around, and especially if her damage is focused on doorways, windowsills, and other thresholds, it is possible that what you're dealing with is not destructive boredom, but separation anxiety.