Learn Danish: Vandreturen i går var anstrengende, men smuk. - The hike yesterday was strenuous, but beautiful.

Anstrengende is a present participle (similar to English words ending in -ing) acting as an adjective. In Danish, adjectives that already end in -e in their base form are invariable. This means it wouldn't add a -t even if the noun were neuter, nor would it change in the plural.

Is it standard to place the time phrase i går directly after the noun Vandreturen?

Yes, exactly like in English, you can use a time expression immediately after a noun to specify it. The entire block Vandreturen i går functions as a single subject, keeping the verb var in the strict second position required by the V2 rule.

Smuk is in its base form. How would the sentence change if we instead said, 'The beautiful hike yesterday was strenuous'?

Moving the adjective in front of a definite noun triggers the Danish double definite rule. You would need the free-standing definite article den, and smuk would take the definite -e ending, making it: Den smukke vandretur i går var anstrengende.

How does the compound noun Vandreturen break down?

It is formed from the infinitive verb at vandre (to hike or wander) and the noun en tur (a trip or walk). Because tur is a common gender noun, the compound takes the common gender definite suffix -en to become Vandreturen (the hike).