Child: "Mummy Mummy, when I grow up I want to play guitar"
Mum: "Don't be silly, child, you can't do both"
Apart from a guitar, you will absolutely need the following items if you're going to spend any time at all with your instrument - and you don't have to spend a lot of money either.
Guitar Tuner - the human ear is no substitute, believe me and while I may not have spent enough time with them, I'm fairly certain you can't train bats to do this most essential of tasks for you. Get a tuner, you'll have more chance of playing in tune with CDs and anybody else.... unless they've used bats.
String Winder - it saves time and energy - although not the sort that will reduce your carbon footprint. They also have a great bit to get the pegs out of your acoustic guitar.
String Cutters - ok, I say string cutters, but you can probably tell they are just wire cutters.
Guitar Strings - I get all my strings from Stringbusters.com because they have fast delivery and very reasonable prices - www.stringbusters.com. Polyweb (sometimes called nanoweb) are fantastic and keep the new sound for longer (although they cost twice the price, they do last three or four times as long.
Guitar Straps - let's face it, you will proably have to stand up and play eventually.....
Strap Locks - ..... and if you're going to have to stand up, you can't afford to drop your guitar. Don't stand up without one.....I mean TWO! One is no good is it ?
Plectrums / Guitar Picks - Let's not forget plectrums - a good plectrum will help you sound great, a rubbish one will frustrate you and make playing more difficult. Steve Clayton has the best plectrums for me (I prefer the Acetal Ones as they not only produce a good tone, but are also virtually indestructible) - there's even an on line design section where you can design your own custom picks, if you're daft enough to spend good money on monogrammed plectrums... I know I am.