By the rule of thumb, from legal perspective - offering cryptos as a prize is the same as offering fiat money. ie. if you're based in US you still can't send cryptos to anyone in restricted country (Iran, Cuba etc) without breaking the law,
So at very least you'd need to construct some T&C and make participants tick to confirm they read it.
It would be useful to know more details on what you're planning to do. What do participants have to do to win? Are prizes of high value? What country are you located?
One of the things we will do is post clues to the location of coins that we have hidden around the world that can be collected through our augmented reality app. Prizes will be small at first but hopefully we'll be able to do some bigger ones in the future. We're based out of the UK.
By the way thank you everyone who responded, it was helpful. In particular I hadn't considered the Cuba, etc. issue before now.