Learn Swedish: Imorgon besöker vi andra städer. - Tomorrow we visit other cities.

Because städer (cities) is a plural noun. The word for other/another changes based on the noun: annan for en-words, annat for ett-words, and andra for all plural words regardless of gender.

Why is the word order besöker vi instead of vi besöker?

This is because of the Swedish V2 (verb-second) rule. The sentence starts with the time expression Imorgon (tomorrow), which takes the first position. The verb besöker must take the second position, so the subject vi is pushed to the third position.

The singular word is stad (city). Why does the plural look so different?

Stad is an irregular noun that undergoes a vowel change in its plural form, similar to man/men or foot/feet in English. The vowel a changes to ä, and it takes the ending -er, becoming städer.

Can I use the present tense besöker (visit) when talking about tomorrow?

Yes! Swedish frequently uses the present tense to talk about future events. Because the word Imorgon already establishes that it happens in the future, you don't need to use a future helper verb like ska, though ska besöka would also be grammatically correct.