Are you gonna be doing retakes or anything throughout this year? Makes a big difference as to how free you'll be...
Just go travelling round Europe...or teach english in Japan? Kinda random...but sounds like fun!
I'd work a ski season, and then go inter-railing.
Work for part of the year, travel for the other part.
If you are short on cash.....do part time jobs (ft is OK if you can manage it)
If you are worried about academics.....do a couple of A-levels
If you are worried about nothing, travel around the world and enjoy yourself
Hmm. You should try and get a job that deals with any of those subjects (easier said than done, I know) or you could try doing something completely different. Consider teaching? Or do you just want to travel? My gap year details were pretty much made as each month went by, but I had a pretty grounded base plan from the beginning, as in a skeleton structure of what I wanted to get done.
I wouldn't recommend using all your savings at once.
If you already have a job, try and work it full time heavy duty for the next month or two to top up on the funds, but first figure out if you want to go travelling just for fun or whether you want to work abroad. If you want to go travelling for fun, I'd say stick with as many jobs as you can take to bring the money and then take a good 4 or 5 months travelling, either with an organisation or with friends (if you're lucky enough to find some with similar plans)
I personally think just travelling to exotic places on a gap year is ok, but being able to live and work in a foreign country to the one you're used to living in is infinitely more useful as you will learn SO much, and possibly even a new language along the way.
If you want to improve your french then perhaps start the hunt for jobs in france (not that easy to find but its possible. My friend was teaching in MArseille and absolutely loved it)
If you are reapplying this year (which I wasn't during my gap as I deferred entry) you may need to think really carefully as to whether you need to do things that are relative to whatever you're applying for.
If you feel like getting into Teaching English as a Foreign Language, PM me and I will be more than happy to chat to you about what you could do.