Tubes cost from between €5 to €10. So it is well worth bringing a spare tube or two. Especially if you are starting out as a cyclist. Easy and all as it is to repair a puncture it is also easy to make a mess of it. So my advice is that if it has been 20 years since you have repaired a puncture, then bring a spare tube and repair the old one when you get home. I have found in the past that bike shops frequently tell you that the wrong size tube will work in a certain tyre, when they don't have the correct size. A word of warning. They Don't! If the tube is too small for the tyre, then when it punctures it is not repairable because it has to be overinflated all the time just to fit the tyre. If it is too big for the tyre then it gets pinched and then punctures more easily. So if you are being fobbed off by your bike shop, stick to your guns and go elsewhere.
Puncture Repair Kits
Even if you bring a spare tube you should still bring a puncture repair kit. I have often gotten two punctures in one outing so bring a repair kit as a backup. There are now self adhesive pads on the market which require no application of glue. You literally just peal the back off them and stick them over the hole.
There is a great range in pumps available now that are only about 8 inches long and clip onto a bracket which attaches to your water bottle mounts on the frame, or you can just throw them into a rucksack. These are ideal as they are light, small and can inflate the tyre to a reasonably high pressure.
If you are really not mechanically minded there are still some other options. One option is to exchange your tubes for self sealing tubes. These tubes are lined with a gel which seals the hole in the tube as soon as it develops. They are heavier than normal tubes. You can also get sealants which come in a canister and inflate the tube or tyre as they seal it. Ask your local bike shop for more information.
You may or may not need tyre levers depending on the types of tyre's that you are using. Some mountain bike tyre's are a very loose fit and will not necessitate the use of levers. Others require three levers two thumbs and a knee! So it is advisable to bring some with you.
It's a good idea to bring some allen keys with you when you cycle you will probably only need 3 or 4 different sizes but they are invaluable if you need to adjust something along the way.
It is also handy to have a link extractor with you. These cost about €15 and give you the ability to repair broken chains. Generally chains break if they are not well matched to your chainset and sprockets. However if they are old or if you have a tendency to use gears that put undue stress on the chain, It will eventually snap. An example of a gear that would put undue stress on the chain would be when the chain was on the largest ring of the chainset and the largest sprocket on the cassette. On some chains it is possible to fit a clip on link. Others require a specific pin which is pushed into position using a link extractor tool.
Food & Hydration
If you are cycling for an hour or more your reserves start to diminish. Bring something with you to eat if you are going out for a few hours. Good snacks include, granola bars, bananas, smoothies, fruit cake, cereal bars. You will also need to drink plenty of fluids throughout the duration. The thing with cycling is that you don't notice how tired you are becoming because it comes on you very gradually. So eat at regular intervals and drink every 10-20 minutes.
Along with water bottles, there are now hydration packs available that look like small rucksacks. They are basically a bag with a long straw that you can use to rehydrate yourself. Some of them contain storage space also so that you can carry all you need on your back.
So where do you put all this stuff?
Cycling jerseys come with pockets at the back so you can either stuff all the gear into them or you can buy a rucksack. There are some great rucksacks on the market that are small and lightweight and perfect for carrying the essentials. You can even fit some rain gear in them if you need to. Alternatively cycling jerseys and jackets are designed with pockets in the back that can carry quite a bit of stuff so you could invest in one of these.