Many objects common in a household can be dangerous and even toxic to dogs and cats. Puppies and kittens are at the biggest risk - they're still learning about the world and investigate everything - but many items can tempt adult cats and dogs as well.
Some examples of items that may not immediately come across as dangerous include:
String of any kind. Ribbons, yarn, tinsel, dental floss, shoe laces, rubber bands, hair ties... All forms of string particularly fascinates cats, but dogs can also fall for the temptation. Some dogs eat rope toys, underwear, or even rugs. One end of the string can entangle itself around the pet's tongue while the rest is swallowed, and it ends up wrinkling the intestines. If you ever see a piece of string sticking out of your pet's bum, don't pull it. Take your pet to see a veterinarian. Even when the entire string or item is swallowed, it can cause internal injuries and blockages. Surgery is needed to save the pet.
Toxic plants such as sago palms and lilies Sago palm usually tempt dogs, and every part of the plant is toxic. Lilies are so toxic to cats that they don't have to chew them - it's enough that they get some pollen on their coat and lick it off. If you ever wonder about a plant, the ASPCA has a comprehensive list of toxic and non-toxic plants.
Medicines, particularly Tylenol and Aspirin Never give your pet a human medication without checking with a veterinarian. Common pain relievers such as Tylenol and Aspirin can be deadly. One pill of aspirin can cause a fatal overdose for a cat. Even small amounts can cause liver damage and kidney failure.
Human foods, such as Xylitol, Raisins, and Alcohol Some people think it's fun to "treat" their pet to beer or other alcoholic beverages. Pets have a much lower resistance to things like that than humans, and may get liver damage. Xylitol is toxic to pets. This didn't use to be a problem since Xylitol was mainly used in chewing gum, but nowadays some peanut butter contain sweeteners dangerous to pets. Other things to watch for include grapes, raisins, and yeast dough. The ASPCA has a list with human foods cats and dogs shouldn't have.
Detergent pods An adult human would never get the idea to eat a detergent pod, but they have proven a danger to both animals and children. They're perfect for batting across the floor and may break when a pet plays with them. Detergent is toxic to pets.
This isn't a comprehensive list, but a good place to start. Try to get on your pet's level and look at the world through their eyes. Items that seem innocent to us can suddenly look enticing!