There are some things that you just cant do without!
As a guitarist now for 6 years a have learned a lot of things about the instrument I play.I used to play bass to in a band so I know some things about it too.Through the years I learnt that there are some stuffs that the guitarists cant leave without and they must always have in their guitar bags1.Picks
This little piece of plastic is the most usable and the most easily lost.Once it falls to the carpet or the floor it tranfert in an parallel universe never to be seems again :-p.The pick is the most important thing any guitarist should always have even in their wallet.You never know when there will be a time to play guitar.Even when you are outside and someone plays guitars you can just take out the your and ask them can I play and you can hit some girls this way :-p.So a must have in your guitar bag.
2.Tuner
A tuner is a must have too.But there are a lot of type of tuners so you can carry one in your pocket every time you want to play outside.So the main types are chromatic and clip on.SoThe chromatic tuner is a simple and a good tuner but the problem is that you need quiet in order to tune or you can plug in your guitar with a cable something you cant always carry with you. In the other hand there is clip on tuner like the one on the picture .This one doesn't affected by the sound of the environment and its the best for live and for playing in the park for example . These days there are a lot of tuning apps for android and ios so you can have one always with you.Even if yu know to tune by ear you don't really know when is the time you must change your tuning.So one in your bag will save you many times.
3.Cables
As I mentioned before you cant carry cables in your pockets but you must have one in your guitar bag all the time.Whatever guitar you play(electric/acoustic) you need one in order so you can be pluged in an amp or a console or anything.
4.Guitar Straps
Another thing that you must always have in your guitar bag.Sometimes you cant play while sitting,for example in band practice and while playing live you cant sit.Sometimes even in your home you want to stand up and play because its way more cool and you fucking love it.
5.Strings
Just always keep an extra string set whenever you go.Strings may cut at any moment..The time that you have them in your guitar is not playing any role although a 6 month change is a must done service for me.Just remember that the strings that you have with you is the same cauge with the one that your guitar is wearing.
5.Stands for guitars(optional)
Whenever you are you need a stand for your guitar and the reason is simple.There is no safe place for a guitar as long its not in your home.It can always drop and break.Once I left my guitar on an amplifier and it was hanging on the wall when suddenly it sliped and fell with the headstock in the floor.The experience was so painful to watch .Thank god my guitar had survived it with nothing but a scratch and I cant even think what would happen if this was the guitar that I bought now omg.But you cant have one in your guitar bag so that's the reason this optional.
That's all.Thanks for reading! :-)