Spare tube:  Sometimes, a tube is just too damaged to be patched, or has had a failure such as the valve stem breaking off which isn’t something that can be patched.   To save weight and space, I usually keep a lightweight version of the right size tube for my bike.   Since one of the first things I do with most bikes that I own is switch them over to heavy-duty thorn resistant tubes, I just take one of the originals and put that in the toolkit.

3D Printers Baby Bath Tub Baby Strollers Bar Stool Bean Bag Chair best actor Bike cover Blackhead Remover Boys' Boxer Underwear Car Chargers Ceiling Fans Chair Cushions cleaner Dining Chair Diving Masks Elbow Braces Electric Air Fryer Electric Stove Fizik Saddles Folding Ladders gas cooktop Handheld CB Radio inflatable boat Kindle Paperwhite Makeup Brush Makeup Chairs Men's Cycling Shoes Paper Towel Holder Ping Pong Paddles pool cue rack Preqnancy body pillow Projector Stands RC Helicopter robot rocking horse Seagrass Rug Small Indoor Dog House sound bars Table Round toddler travel bed vacuum Vanity Makeup Mirrors Walmart Folding Table Women's Condoms Wood Table Lamps
2) On my second day, the pannier bag fell off and got lost after I stopped for lunch, which required locking up the bike somewhere, so I took the bag into restaurant and re-attached it using the two velcro straps on the rear rack. I've used many panniers before, thought I did it correctly. But apparently I did not. So I paid $55 (cost) to replace everything in bag and bag. In hindsight, I wished I'd put a bungee around bag straps to really secure it to bike. PROBABLY MY FAULT.
Keychain style flashlight or headlamp, LED type, not incandescent:   The keychain style is tiny and lightweight, the headlamp style is a bit bulkier but easier to use.    You can get these for a couple of bucks or less at the local big box hardware store.  The LED ones are lighter, more durable and last considerably longer.   Very useful if you need to make a repair after dark!  I keep a keychain style one as a backup as I normally have a headlamp in my backpack.
Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.
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