My top choices if you only have a short amount of time in Milan!
Most people go to Tromsø to see the Northern lights, but there's a lot more to see here.
Formentor is a spectacular national park on the island of Mallorca, Spain. If you want to explore the island, it's definitely a great stop for your mini roadtrip.
Ljubljana is one of my all time favorite cities - super fun, artsy, beautiful, and historic.
If you're in Trieste, Italy or Ljubljana, Slovenia, then Postojna Cave is a great day trip!