W weekend planujemy krótką wycieczkę w góry.

W weekend means “on/over the weekend” or “this weekend”.

• w is a very common preposition meaning in / at / on (depending on context).
• With days and some time expressions, w + accusative is used to mean “on [that day/time]”, e.g.
  
  • w poniedziałek – on Monday
  • w sobotę – on Saturday
  • w weekend – on/over the weekend

So w weekend is the normal, everyday way to say “at the weekend / on the weekend” in Polish.

Yes, there is a nuance:

• w weekend – focuses on when something happens:

  • W weekend planujemy krótką wycieczkę w góry.
    We are planning a short trip to the mountains (during the weekend).

• na weekend – often means “for the weekend (for its duration)”, and is especially common with verbs of movement:

  • Jedziemy w góry na weekend. – We’re going to the mountains for the weekend.
  • Wynajęliśmy domek na weekend. – We rented a cabin for the weekend.

In your sentence we’re talking about what will happen during that time, so w weekend is the natural choice.

In Polish, the present tense of an imperfective verb (like planować) is often used to talk about:

• planned future actions
• timetables, arrangements, fixed plans

So:

• Planujemy krótką wycieczkę w góry.
  literally: We plan / are planning a short trip to the mountains.
  but in context it’s understood as a future plan – something that will happen.

English often uses “we’re going to…” or “we’re planning to…” for this idea; Polish comfortably keeps it in (grammatical) present tense.

This is about case and agreement:

• The basic (dictionary) form is:
  
  • krótka wycieczka – short trip (nominative, feminine singular).
• In the sentence, wycieczkę is the direct object of planujemy, so it must be in the accusative case.
• For a feminine noun ending in -a, the accusative singular ends in -ę:
  
  • wycieczka → wycieczkę.
• The adjective must match the noun in gender, number, and case, so:
  
  • krótka (nom.) → krótką (acc.).

So krótką wycieczkę = a short trip in the accusative case.

• wycieczkę is accusative singular, feminine.
• The nominative singular (dictionary form) is wycieczka.

Pattern:

• nominative: (ta) krótka wycieczka – (this) short trip
• accusative: (tę) krótką wycieczkę – (this) short trip (as object)

In the sentence, planujemy [co?] krótką wycieczkę – “we are planning what? a short trip” → direct object → accusative.

Both can translate to “trip” or “journey”, but they’re not fully interchangeable:

• wycieczka

  • Often: a shorter, usually leisure trip (excursion, outing).
  • You might say:
    
    • wycieczka w góry – a trip to the mountains
    • wycieczka szkolna – a school trip
  • Suggests something relatively light, often planned for fun.

• podróż

  • More like journey / travel in general.
  • Can be long, serious, or just neutral travel:
    
    • długa podróż – a long journey
    • podróż służbowa – a business trip
    • podróż do Japonii – a journey to Japan

In krótką wycieczkę w góry, wycieczka nicely matches the idea of a short, probably recreational trip.

The difference comes from motion vs location:

• w góry – into/to the mountains (motion, direction)

  • accusative plural → movement towards a place
  • common with verbs like jechać (go by vehicle), iść (go), jechać w góry, jechać nad morze.

• w górach – in the mountains (location, where something is)

  • locative plural → being in a place
  • e.g. Mieszkam w górach. – I live in the mountains.

Your sentence describes planning a trip with the destination in mind, so the standard phrase is wycieczkę w góry – “a trip to the mountains”.

In w góry:

• góry is accusative plural (same form as nominative plural: góry).
• With w + accusative, it expresses movement into somewhere: w góry – into/to the mountains.

Other common forms:

• w górach – locative plural:
  
  • with w + locative = in the mountains (location, not movement).
• gór – genitive plural:
  
  • used for “of the mountains”, “from the mountains”:
    
    • widok gór – view of the mountains
    • wracamy z gór – we are coming back from the mountains.

So:

• w góry – to/into the mountains (direction)
• w górach – in the mountains (location)
• z gór – from the mountains (origin)

Polish word order is fairly flexible, especially for adverbial phrases like w weekend and w góry. All of these are grammatically possible:

• W weekend planujemy krótką wycieczkę w góry.
• Planujemy w weekend krótką wycieczkę w góry.
• Planujemy krótką wycieczkę w góry w weekend.

Typical tendencies:

• Putting w weekend at the beginning sets the time frame as the main context:
  W weekend planujemy… – As for the weekend, what we are planning is…
• Leaving w góry near wycieczkę keeps “trip to the mountains” tightly together, which sounds very natural.

All three versions would be understood; the original order is very neutral and natural.

You can, but it changes the nuance:

• Planujemy krótką wycieczkę w góry.

  • imperfective planować in present
  • implies we are (already) planning / we have it in our plans now; the plan is in progress or basically decided.

• Zaplanujemy krótką wycieczkę w góry.

  • perfective zaplanujemy (future)
  • means “we will plan a short trip to the mountains (at some point)” – focusing on the act of planning as a completed future action.
  • could sound more like a promise or decision to make a plan later, rather than saying the trip is already being planned.

In everyday speech, if you mean we already have this plan for the weekend, Planujemy krótką wycieczkę w góry (w weekend). is more natural.