Aged 15, I spent one two-week school holiday doing nothing apart from playing computer games. At the end of it, I felt pretty terrible: I looked back and realised I had wasted two weeks and got nothing out of it. I resolved to do better next holiday, and the following holiday I ran 3 miles every day for two weeks. To begin with, it felt like a chore, but towards the end I was enjoying it. That was definitely better than the wasted holiday, but looking back I could've done better. The point of this post is firstly, to encourage you to use holiday time to train, and secondly, to try to give some tips on how to train well.
I'm sure if you sat and thought about it, you'd agree that it would be beneficial to exercise - it probably isn't too much of an exaggeration to say that during holidays we waste a lot of time, and I know I could easily fit in a lot more than I currently do. So what are you waiting for - do it! A couple of weeks of decent conditioning will mean that when it comes to parkour next term, you can spend more time learning and improving and less being tired. So use your will power. Don't procrastinate - if you're watching TV or on your computer and have nothing better to do, do something right now! Or put it as part of your schedule in the morning, either before or straight after breakfast.
So, now that you've got off your bum to do something, what do you do? Whatever it is, make sure you're training smart (or smartly, for those who follow rules of grammar) - at this point my example of running 3 miles each day was not such a good one. Firstly, mix it up: don't repeat the same thing, as while it will be beneficial, the benefits are limited, and without variety it's harder to continue improvement once you reach your first plateau. If you want to get fitter, don't just do continuous running: do interval running, vary your speeds, etc. This applies both to speed work and endurance work (there's more variety than that, but we'll keep it simple). So an example of different sessions that a distance runner might use: 30 minute continuous run, 20 minute fast run, 5 x 2 minute runs (2 minutes break between), 15 x 200m runs (but break it up, so 3 lots of 5 or 5 lots of 3, with a certain time between each 200m - can be 30 seconds or 2 minutes depending on the speed you want to run!), and so on. The same is true for speed stuff: sprinters do sessions which range from 20m to 600m with a variety of recovery.
But we don't simply want to get good at putting one foot in front of the other: this is true for all exercises. For press-ups (for example, a staple upper-body exercise), don't just do as many as you can every day in one sitting - yes, this will help, but a smarter workout would help more. Mix it up between explosive press-ups (for example clap press-ups, but if that's too hard then just try and get a bit of airtime) and slow ones (taking 10 seconds per press up). Make it so that on some variations the most you can do in one go is 3, and on others go up to 15. Also repeat it - don't just do one lot, but give yourself a few minutes rest and go again. So, an example: three sets of press-ups with 5 minutes inbetween, in each set do as many of a certain type as you can. Another way of making press-ups harder is by raising your feet (incline press-ups) so that more bodyweight goes through your arms.
Make sure you're working towards a certain target too. If you want to be explosive, do exercises that work muscles explosively. If you want improve your endurance, do exercises that work that, remembering that what you're working is endurance at a particular strength. For example, if you want to improve your strength, don't do 20 press-ups, as this works the endurance more (it will improve your strength, but not as effectively as something targeted). Instead, find an exercise where your maximum is around 5.
Mix up your exercises too. Don't do standard press-ups all the time - I can think of around 15 different variations on press-up that work the same areas of muscles, but in different ways. Wider or lower hand placement or uneven hand placement, for example. Doing panthers/dive monkeys (same thing, two names - google it!) are also good for this.
As well as mixing up your exercises, make sure you're working the whole body - legs, core (both abs and back), chest, arms, and upper back.
I'm not going to list exercises or plans - there's plenty of information out there - but if you are stuck feel free to ask me anything.
Aside from standard conditioning, get out there and practice parkour! I improved my balance a lot simply by using the road sign at the end of my street - every time I went past it I would jump on it. Either two-footed facing it, taking off one foot facing it (alternating feet), or running from behind it and trying to jump on and carry on running. Once you're on it, walk up and down, turn around, try standing still facing forwards, or even do squats (or one-legged squats!) to practice balancing. Similarly, there's a lot to be done on a simple kerb or wall. Doing ten minutes regularly will make a noticeable difference! Again, if you're out of ideas, search the internet or ask me.
This has dragged on a bit, but hopefully you can see there's a lot you can and should do!