There are three types of "giveaways":
I know that this line is very fine and tricky to navigate.
The biggest factor I have found is that everyone needs to be able to participate with little to no barrier. The safest thing to do is have something in the fine print, a website or a place they can go to enter the sweepstakes without having to jump through the hoop you are presenting.
So present the sweepstakes as you want, but in all the fine print you put something like - no purchase necessary, submit your entry at xyz.com/win ... yada yada.
I'm not a lawyer, but this is often what groups do and to my knowledge it is acceptable.