Learn Hardcore Norwegian: Turen i skogen gjør meg mindre stresset. - The walk in the forest makes me less stressed.

If you said tur i skogen without -en, it would sound like you’re talking about a walk in the forest in a more general way, not a specific one.
Both are grammatically correct, but turen is more specific:

En tur i skogen gjør meg mindre stresset. = A walk in the forest makes me less stressed.
Turen i skogen gjør meg mindre stresset. = The walk in the forest makes me less stressed. (a specific or more concrete walk)

Why do we say i skogen and not til skogen?

i means in / inside.
til means to (movement towards a place).

i skogen = in the forest, describing where the walk happens.
til skogen = to the forest, describing movement towards the forest.

So:

Jeg går til skogen. = I’m going to the forest.
Jeg går en tur i skogen. = I’m going for a walk in the forest.

In your sentence, the focus is on the walk that takes place in the forest, so i skogen is correct.

Why is it skogen and not just skog?

Same reason as with turen: the -en ending makes it definite.

en skog = a forest
skogen = the forest

i skogen = in the forest (a specific or understood forest)
i skog is not normally used in this context; you almost always say i skogen.

Everyday Norwegian likes definite forms with nature words like this:

på fjellet = in the mountains / on the mountain
ved sjøen = by the sea
i skogen = in the forest

What exactly does gjør mean here?

gjør is the present tense of å gjøre = to do / to make.

In this pattern, gjøre is used like English make:

noe gjør meg glad = something makes me happy
det gjør meg trøtt = it makes me tired

Your sentence:

Turen i skogen gjør meg mindre stresset.
= The walk in the forest makes me less stressed.

The structure is:

[subject] + gjør + [object] + [adjective]

So: turen i skogen (subject) + gjør (verb) + meg (object) + mindre stresset (adjective phrase).

Why is it meg and not jeg?

Norwegian has different forms for subject and object pronouns:

jeg = I (subject form)
meg = me (object form)

In this sentence:

Turen i skogen is the subject (the thing doing the action).
meg is the object (the one being affected).

So:

Jeg går en tur. = I go for a walk. (jeg is the subject)
Turen gjør meg glad. = The walk makes me happy. (meg is the object)

Therefore, gjør meg (makes me), not gjør jeg.

What does mindre mean, and how is it different from mer?

mindre means less, and mer means more.

They are used to compare amounts or degrees:

mer stresset = more stressed
mindre stresset = less stressed

Examples:

Jeg blir mer stresset på jobb. = I get more stressed at work.
Jeg blir mindre stresset i helgene. = I get less stressed on the weekends.

So mindre stresset is a comparative phrase: less stressed.

What kind of word is stresset here? Is it a verb or an adjective?

Stresset here is used as an adjective, describing a state: stressed.

It comes from the verb å stresse (to stress), but the past participle stresset is very often used as an adjective:

Jeg er stresset. = I am stressed.
Hun virker stresset. = She seems stressed.

In your sentence, mindre stresset describes how you feel:

gjør meg mindre stresset = makes me less stressed.

Colloquially you might also see stressa (with -a) instead of stresset, especially in speech or informal writing:

Jeg er stressa.

Both are understood, but stresset is the more standard written form.

Could I say En tur i skogen gjør meg mindre stresset instead? What’s the difference?

Yes, that sentence is also correct.

En tur i skogen gjør meg mindre stresset.
= A walk in the forest makes me less stressed. (more general)
Turen i skogen gjør meg mindre stresset.
= The walk in the forest makes me less stressed. (a specific walk, or something more concrete/known in the context)

So en tur = a walk (indefinite), turen = the walk (definite).
The grammar is the same; only the nuance of specificity changes.

Is the word order gjør meg mindre stresset fixed, or can I move things around?

The natural word order is:

gjør + [object] + [adjective phrase]

So:

gjør meg mindre stresset (correct and natural)

You cannot move meg after the adjective phrase like this:

✗ gjør mindre stresset meg (incorrect)

You also cannot put meg before the verb in this main clause:

✗ Turen i skogen meg gjør mindre stresset (incorrect)

In a main clause, the typical pattern is:

Subject – Verb – (Object) – (Other information)

So:
Turen i skogen (subject) – gjør (verb) – meg (object) – mindre stresset (description of the object).

Why is gjør in the present tense here? Could I use the past?

gjør is present tense and is used for:

general truths / habits
things that happen regularly or typically

So the sentence is like saying:

Walking in the forest (in general) makes me less stressed.

If you want to talk about one specific walk in the past, you can use the past tense gjorde:

Turen i skogen gjorde meg mindre stresset.
= The walk in the forest made me less stressed. (that time)

Both are correct; it just depends whether you’re talking about a general effect (gjør) or a past, specific situation (gjorde).