So far the our only breakdowns have been punctures, a broken chain, minor adjustments and things you can't fix without a workshop so I only take a few basic tools:

• allen keys of appropriate sizes
• a spanner to remove the pedals for transport (you can leave it with the bike box/bag)
• a leatherman tool, the pliers are useful
• a puncture repair outfit and a couple of spare tubes
• tire levers
• a multifunction tool that includes a chain repair tool
• a couple of chain repair links and a few links of chain
• hand wipes to clean your hands

I've never used the puncture repair outfit, just swapped tubes, but the glue has a shelf life so replace it before every trip. Also a crayon that will work on the tube is useful.

I'd suggest practicing using chain repair tool before you leave, until you are proficient. Trying to work it out by the the side of the road is no fun.

A good way to avoid punctures is to keep your tires pumped up to the correct pressure. Most pumps are designed to get you home and won't achieve this. I find the pumps where you have to hold the end on the valve hopeless, you can't do that and apply the force you need to get the pressure. My pump has a flexible tube that you attach to the valve. It also has a little flap you can flip down and put your foot on.