I am going to explain my point twice, YOU choose which one fits you best.
A) Since I'm a sophmore, and I'm nineteen years old, I have personally been saving up since I was much younger. At fourteen, I stopped spending every penny I would get, and started to save up by opening my first personal bank account (with a cosigner-my parents). If you are at about this age, then this is great, because let me tell you one word of advice, before talking about investment ideas. Save as much money as you can, starting today. If you are going to be paying for college, like I am, press down hard on the pedal, go to your local restaurant or pizza restaurant and get a night job on Friday's, Saturday's or Sunday's (or all three :)). I have held the same job as a busboy at the local pizzeria in my small town of Croton, NY for the past five years. I work friday and saturday nights (only summers during college). For this post, you should realize that your main concern for now is to get a job, or job(s) during the school year. I would also strongly recommend that the job is on Friday, Saturday or Sundays during the school year since risking your focus on school work is the poorest investment into your future that you could ever make. Don't make that mistake. Your education right now is worth more than the million's you will make over your lifetime.
B) If you are eighteen years or older, and you are paying for college (with student loans), then you probably have been working hard this summer (I hope). Even if you aren't paying for college, I hope you enjoyed your summer, but at the same time worked hard. You will definitely need money for this following year. Personally, I have worked extremely hard since I was fourteen because I knew that I wanted to go to a University away from home, and I also knew that I didn't want to have to work a lot during college, because like I said in "A)," education is the wisest investment one can make and risking it could cost me more in the long run.
If you haven't been saving, and you have to pay for college, don't give up! It's not too late to make a strong turn around. You are already ahead of the game. The only question I would have, is...how the heck did you find this blog? You have no summer money, but your looking to double it? Um, okay, I guess you are planning for next summer! Great then, your WELL ahead of the game. Cheers....Anyways, it really is not too late. Get a night job during the school year! Being a waiter, waitress, or a busboy is a great way to make cash money (most times) and plus, you usually make more than minimum wage (after including tips). Plus you get food...for free most times :D. Any other job would work as well. Money is money.
So what should you take out of this post. No matter what position you are in. Under eighteen, or over, you should be working hard and saving money. That's the only way you will ever GROW YOUR SUMMER MONEY.
~Organization is key when investing wisely (its key in every aspect of life)~