Personally, I would like Charity Donations/Fundraising as YourFutureEx is a total non-profit project. I am here to help and the rewards I get will eventually be distributed to the people directly or indirectly.
In fact, as a victim of domestic violence, RAVEN was one of the inspiration for me to sign up here. It was great that I could help needy people by just having fun on a website. It could be anything whether that organization which brings clean and safe drinking water to people in developing nations or something like RAVEN or any other NGO which inspires us to take a step.
Not everyone here is after rewards, give those people an option who wants to donate.
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From community point of view, first of all, I'd like you to expand your available options for Amazon gift cards. Amazon gives them the opportunity to buy whatever they want (option A, B, C, D). It is the most efficacious option as it covers everything. You have the stats. You know from which countries you are getting good traffic. Introduce Amazon gift cards for their countries as well. That way, we could share our gifts with each other (option E as well). Most G@Gers will get benefited.
I don't know what's popular in the US, but you could introduce another popular online shopping options like ebay (for the same reason I advised you to expand Amazon options) as it will probably in $20 range so, we'd have options to buy things. While you can't really buy lot of things on Steam, Playstation Store & Microsoft store, at least there are some 'exclusive' thing we cannot buy elsewhere. Clothing discounts would be a great option & you can buy food and books in that range tho. But e-commerce companies cover it all so, what's the point?
Google Play & iTunes gift card will be lucrative option for you as well as you are promoting your app, you would find a lot of 'smartphone users' who are interested in tech stuff related to smartphones and you could lure those people to use your app and get rewards. You could get lot of apps, movies and songs in $15-20 range. That seems reasonable.
Good luck :)