Ditty bag – A rather old fashioned term for a small bag used to carry small useful things around in, not as big or as purposeful as a rucksack, daypack, handbag, etc.
“I’ll just have a look in my ditty bag and see if I’ve got a pen.”
“Put those papers in your ditty bag or they’ll get lost.”
Doggy bag – very specifically the bag of leftover food you ask for in a restaurant when you can’t finish your meal. Rather than let it go to waste, you ask to take the remains home, implicitly to serve it up to your dog. Of course, you may well be planning to have it for tomorrow evening’s meal. So, the waiter takes the plate out to the kitchen and has it wrapped up for you to discreetly carry with you when you leave. A popular practice if you are on a tight budget.
“We’ll never get through this mountain of food! Let’s ask the waiter to prepare a doggy bag!”