It's perfectly appropriate to ask for more time to consider an offer, as long as the extension you're asking for is within reason. Of course, you should always be prepared for the possibility that they might say no. It's also a good idea to mention the reason why you're asking for an extension, assuming that reason doesn't reflect badly on you or the company.
In your case I would ensure that your email contains the following:
- express thanks for the offer: as Frisbee says this is quite the compliment
- explain that you're inclined to accept (assuming that is the case)
- explain your reason ask for an extension: "given how hectic things are while I'm wrapping up my final week I wanted to ask if it would be okay if we pushed the acceptance deadline an extra week so that I have enough time to make sure that I'll be available."
- acknowledge that the deadline might be set in stone: "Of course if that's not possible on your end I'll make sure to get back to you before Friday/the Xth"
Now, I suggest bringing up that you're checking your availability for next summer because that gives you an alternative reason to ask for an extension. That's because no matter how you look at it, the fact that you need additional time to consider means that you're not completely sure that you'd want to return. If you love the work, look forward to coming back, don't have an alternative offer coming up in the near future and don't have a scheduling conflict for next summer then I would recommend simply accepting.
Note that a reasonable manager won't think it weird that you're not entirely sure yet. In fact, if you were asking for an extension face-to-face a good manager would probably ask you what concerns you have over coming back and try to work out what both of you want to get out of your next internship. I would actually recommend that you have this exact conversation with him before accepting as it's a good way to get aligned on your goals for next summer. For instance, if you felt like you were only doing low-level work and would like an opportunity to work on some higher-level stuff then that's something to bring up. It's not unreasonable to ask those questions when you're offered a second internship.