On any ride a spare chain quicklink is so light in relation to its potential usefulness it seems churlish not to carry one.
On a longer tour (or in a group where the load can be shared) its also worth considering carrying such items as;
- spare spokes (+ whatever tools you need to fit them)
- cone spanners (or whatever you need to be able to fettle the hub bearings in your hubs)
- spare rear axle (if you use a screw-on freewheel)
- whatever tools you need to adjust parts on your bike; for example I have habitually carried something to retighten my headset locknut on bikes with a threaded headset.
- spare folding tyre
- at least one M5 bolt that is long enough to refit a derailleur pulley if necessary (often you can swap them about so that the replacement bolt fits in the lower position even if you have lost the top one and it needs to be a special head shape)
- spare derailleur pulley including the bushing (if one falls out you may find the pulley but not the bushing)
- at least one M5 bolt that is the correct type for cleats
- at least one M6 bolt that is long enough to work in a HTII crank and/or as a seat binder bolt
- a few turns of PTFE tape
- a few turns of insulating tape
- a few turns of gaffer tape (tapes can be wrapped around something handy)
- spare bulbs (if your lights are not LED ones)
- a short length of heat shrink insulation for doing wiring repairs (may also be used to replace cable tidies)
In the special tools category;
- freewheel/cassette lockring tool
- tool to retighten/remove a freehub body
- headset tool
- BB tools/modifications
- torx key for brake disc bolts/FPA adjuster on BB5/7
- SPD pedal bearing tool
- pliers/adjustable spanner for gripping/bending stuff with
- a 4" length of old screwdriver shank, which can make a useful drift.
I have a short hollow 10mm allen key for freehub bodies and a small Ti spanner to fit my headset locknut. They weigh about 35g together so are not exactly burdensome to carry, even though experience tells me I won't actually need them....
PTFE tape can work as something that will prevent a loose screw (or one that will loosen) from backing out and being lost. For example if a crank comes off, using PTFE tape on the crankbolt may prevent it from backing out entirely.
Regarding BB modifications/tools; if using a HTII style bottom bracket, it is worth considering drilling a couple of small holes in the outside of each cup; this will make it possible to retighten the cups (using a drift) should they come loose, well enough at least to be able to carry on until you can get proper tools.
If you have to rebuild a bearing 'in the field' then there is a variety of things that will substitute for grease temporarily, including butter, sun cream, vaseline, lip salve.... Because there is a fair chance of one or more steel balls being damaged (which is why you took the bearing apart...) or lost it is worth carrying a few assorted steel balls to make good any losses. Half a dozen of each of the common sizes ought to be enough to make good temporarily. A 35mm film canister can store an awful lot of these smaller items.
Roadside repairs fall into two categories;
1) repairs that are permanent and as good as a workshop repair and will last indefinitely
2) temporary repairs that will last the day or the week but are not 100% satisfactory. These are to be used to get you to the next town or the next LBS.
In general very few of the roadside problems I have encountered (in my own bike and others) couldn't have been forestalled by better maintenance beforehand. "Proper Preparation Prevents P-Poor Performance" and all that. But knowing that at all and in particular where best to apply oneself beforehand only comes with (often bitter) experience.