Mystery bags have changed up rewards a bit and there is now a "Red Mystery Bag" available for gifting.
Here are some of the rewards I have gotten:
+100 Energy Points
+1 free reward point
+5 Free Reward Points
+5 Skill Points (best!)
Valentine's Day Heart Tattoo (previously was quite rare)
Misc. assorted boosts
For some reason I am unable to receive any more mystery bags and every time I try to accept one it says "you have already accepted the Red Mystery Bag from that person" even though I was only able to open a few bags before getting this message.
At any rate, feel free to post what you find and I will update this list. Thanks!