I just got back from a trip to London. My travel purse, which I carried every day in London (my husband had a small day pack) contained:
-Oyster card (that's the London transit card)
-a small wallet, containing US driver's license, ATM card, 2 credit cards, health insurance card, and Royal Historic Palaces member card
-some cash (always some change in case of a pay toilet)
-a small Tube map, the paper kind they give you
-feminine hygiene products as needed
-a couple of band-aids
-a snack-sized Ziploc bag with a tiny container of hand lotion and lip balm
-a small plastic container for any meds I might need that day, plus Tylenol and ibuprofen just in case
-my set of keys to the flat we'd rented
-one of those skinny Kleenex purse packs
And that's it. My husband carried the map, list of information, water bottle, pencil, snacks, umbrella when needed, a plastic carrier bag (plastic grocery bag), and whatever else he had (mostly stuff we'd picked up that day, like a gallery map or the odd purchase) in the day bag.
My travel purse is a $20 black fake leather one I got at Target. It's smaller and lighter than any other purse I own, has an outside zippered pocket for my travel card, and one main zippered pocket for everything else. I can wear it cross-body (this is more for not having it slip off my shoulder or leaving it behind, than worrying about theft) and did I mention it's light? It was inexpensive enough I wouldn't care if anything happened to it. I found out on this trip that getting soaking wet in the rain doesn't hurt whatever it's made of!
Note: for London, I didn't bother with hand sanitizer and extra Kleenex for emergency toilet paper. In other places, I have and would again!