Best type of contest / giveaway for follower benchmarks

Hello beautiful people!

I am running various instagram accounts, most are nature photography, hiking, outdoorlife niches. I plan to take advantage of the fact that one of them will soon reach 5k followers and I will do a giveaway or a contest - I am not sure yet which one I’ll choose.

What is your best experience with contests or giveaways? Is one better than the other? What did the followers have to do if it was a contest? All ideas would be much appreciated. I really want to take this chance and grow my community even more (this would be the main goal) but at the same time offer something fun and interesting to my existing followers.

Thanks in advance!