Help me finalize my itinerary! September/October Visit

*Hello! I am finalizing my trip to Europe and would absolutely love to have the input from this subreddit.

*Background info: I am 28M, solo travel for ~3weeks from USA. I have been to Europe 6 times previously, but with family or friends. I am excited but nervous to travel solo. My dates are currently September 18-October 10. I will be staying in hostels and using public transit whenever possible.

*I am hoping that you all can provide me ideas for the following items: Activities, Restaurants, Coffee, Museums, Sights, Accommodations.

*September 18 9am - September 21st 5pm - Reykjavik

*September 21st 10pm - September 26th 9am - Copenhagen

*September 26th 11am - September 30th 9pm - Budapest

*September 30th 11pm - October 5th 6:40pm - Stuttgart

*October 5th 8pm - October 10th 10am - Split

*October 10th 10am depart for London, then back to USA.

* The only place i am unsure if is copenhagen. Please let me know your thoughts! thank you all so much