You could watch ads inside the 3 lines section. There would be a section called: Ads
You could watch an ad to support the devs and to keep the servers online. A leaderboard would be great for supporters inside the app. A top 1000 leaderboard would be great for the most active supporters. On the leaderboard (no matter if it's the top 1000 or the full) you could see your and everybody's watch count,ad click trough count and the made money. I think this would be great, for example 1 ad watch is 0.75 cents, and a click trough is 1$ than 100=100$. Just calculate with these numbers and you will see that money income will grow potentially.
One important thing: If you consider adding ads, NEVER do it like in arcade games. Don't make ads pop in and such, just make an options for those how want to support but don't have money or credit card.