so this means both "where are you going" and "what is the purpose of this trip"?
well could you be asked this phrase at the border control for example? like what is the purpose of the trip
then it would be "destination". Wycieczka is "tourist/sightseeing" by default.
jaki jest powód/cel państwa podróży = what is the purpose of your journey- would be what you would be asked by border control.
"Jaki jest cel tej wycieczki" may be a question a teacher who wants to take his class to the zoo/museum would be asked.
I used the commonly used word 'outing', which was rejected. The 'Wieki slownik' lists this as the second meaning of 'wycieczka' after 'excursion'.