There is a ton of things I could recommend if you are interested in history, but it will be hard to squeeze them into one day of exploring. But Gamla stan is a good start, with the parliament close by on a seperate island. If you are there during lunchtime there is the changing of the guards at the Royal Palace every day at 12.15 (13.15 on Sundays and holidays).
When you are done in Gamla stan, take the ferry to Djurgården and visit the
Vasa museum. A nice museum, not only about the ship but about life in the 17th century in general.
There is a good chance the train staff won't mind if you stay in the train and alight at Umeå C instead. If the weather is good, the walk along the river is nice. But in case of rain, or worse (sleet, hail or snow is possible in april), the train is nicer.
If you need something to do in Umeå, the art museum is worth a visit. And Kafé Stationen is a good place for a fika.
One hour around midnight in Örnsköldsvik will probably not be the most exciting part of your trip, but it will give you time to check out the railway station that is built over the lower part of a ski jumping hill.