I have built https://audiala.com which creates audioguides for historical and touristic places in cities all over the world. It brings a bit over $500/month in in-app purchases. I got the idea in 2023 as I was solo traveling Florence, Italy and thought it would be much nicer to listen to stories about the monuments around me instead of having to read a guide. There is also so much more to be done: next, my plan is to create personalised itineraries based on your preferences, starting point, etc. I tried paid marketing but found much more effective the SEO I have done on the website, and users seem to share with their friends and come back, which makes me happy.