выезд на природу

[Fortnight]

выезд на природу с семьей

[dishadisha7]

http://multitran.ru/c/m/a=3&s=trek to the country&sc=0&l1=1&l2=2 возможно это либо Field trip как вариант

[eu_br]

family retreat

[Oo]

http://en.wikipedia.org/wiki/Picnic

[lisulya]

family outing

[Wolverin]

Try:
Family picnic
(i.e., an excursion or outing with food usually provided by members of the family and eaten in the open)