Make them gig out something underappreciated and give them a reward for that. I once did an SBI giveaway for recommending underappreciated hidden science fiction gems (movies). It worked extremely well in terms of the responses. You can check it out here: https://steemit.com/steembasicincome/@vimukthi/share-your-underappreciated-science-fiction-hideen-gems-movies-and-win-steem-basic-income-shares Hopefully you'd find a few nice picks for you too.

I've also given bounties using @steem-bounty for engaging in specific acts. This is a great way to get people involved or just promote some content or author you find amazing.

I've also given SBI to those who have ended up signing upto STEEM DAPPs using my referral links as a way for showing appreciation.

I've seen some great charity projects on https://fundition.io You can ask people to donate or upvote them to qualify for the giveaway.

Then there is always the riddles and puzzles you can try which can be quite fun and even educational.

Best of Luck!