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.
The best type of bike for exercise depends what level of rider you are and whether you would rather ride on trails or on the road. There is a type of road bike that is called a “Fitness Bike,” and features a lightweight frame and narrow tires, but do not have a drop handlebar. This may be the best type of bike for someone who intends to ride regularly and strenuously on the roads. If you plan to do a lot of trail riding, the best bike for exercise would probably be a mountain bike. For a person who uses bikes for transportation and exercise, but doesn’t go off-road, the hybrid bike build is a good type of bike.
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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