As promised, there was a raffle amongst participants of the app.
I wrote a script in the program that I use for data analysis (R in RStudio) that randomly picked 10 participants by their user id and then matched that to their email address, which was stored elsewhere. The first five won the Artis tickets and the next five won VVV giftcards.
Congratulations to the winners:
Of the Artis tickets: Hugo, Bogdan, Ingrid, Mariken & Ioana!
VVV giftcards: Tamara, Christiaan, Enrico, Bouke & Martina!