Not to excuse you for your behavior, but if your employer hasn't given you feedback on your performance, then they are partly (but in the minority share) to blame. Internship is in part about learning how to be an employee, and those who take on interns take on the responsibility of helping to develop that employee. Have you received no feedback from them whatsoever? How long have you been there?
Really, trying to return money or not cash the check is not going to help anything and will likely make it quite worse, because then they'll see you not just as useless but also annoying or even a little crazy.
How much longer is your internship for? If it's open-ended, or will last a while longer, then you stand a chance to improve. Just don't engage in behaviors you know are wrong. It's not that hard. If it is that hard, you have problems that we can't solve for you. If you're close to the end of your term, and they're just waiting for you to get the hell out of there, then as other have said your consequence will be a poor evaluation that will hurt your chances of future employment.
What exactly do you consider "slacking"? Why is "don't be a slacker" difficult advice to take? If by "slacking" you mean you actively waste the company's time engaging in non-company activities like recreationally surfing the web, then just DON'T DO THAT.
If, instead, you feel like you do spend all your time working but your productivity isn't what you think it should be, or you're just not as good at it as you think you should be, that's really no big deal. That's what being an intern is about--you're a beginner and you're not going to be an instant expert at what you do. If that's all the problem is, then you are *not* in any trouble with your boss, and you just need to go to them and ask them for suggestions for how to improve.
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