Here are some things to consider when you're looking at what internship offer to accept:
Is it for a company you like? Can you see yourself working there full time? Is it doing a job you would enjoy?
What skills will you build by working there? How do these skills relate to your longer term career plans?
Now, when they offer you an internship, some companies won't tell you what you will be doing over the summer, primarily because even they don't know yet! My company, for example, was still figuring out internship assignments in March of this year, when we had extended our offers the prior fall.
So how do you choose in that case? Well, look specifically at the company to see what they offer, what industry they are in, and whether you like the idea of working there full time afterwards.
There is also a second bit of negotiating or contracting you can do with them (though, I must caution you: do this very tactfully, do not be pushy or arrogant!)...
That is to let them know that you are considering multiple offers, and that you want to make an informed decision. Tell them that you understand they may not have a specific job in mind yet, but that there are certain elements to the internship that you want to see (See how this theme of job elements keeps coming back!). Let them know what 3 or 4 specific elements you are looking for, and ask them if they can tailor your assignment to fit those elements.
Interestingly enough, the program coordinators do spend a lot of time looking at each candidates strengths and weaknesses versus the internship opportunities the company has, with the intention of getting good parings. If you have these specific requests, it is not that much more difficult for the coordinator to take that into account when doing assignments.
Again, I have to emphasize that you cannot be pushy about this, and it can't come off as a "give me this or I walk" attitude. It has to be a partnership of "I look forward to providing my valuable services to you this summer, here are some things I hope you can provide me in return."
