"Cały dzień chodzimy po plaży" sounds like Present Continuous to me ;]
They are not interchangeable at all. The problem is that the sentences in this skill have almost no context at all.
Check the Tips&Notes for "Present 2" (the lightbulb button after you click on the skill, at least in the web version) or this article: https://www.clozemaster.com/blog/polish-verbs-of-motion/
a) mężczyzny is Genitive from mężczyzna.
b) mężczyzna is one of specific nouns that is masculine but ends with "a". those nouns have the same declination as female -a ending nouns, including different Accusative and Genitive. (but heve male pronouns, numerals, adjectives, verbs)
c) we use Genitive after " do"