~に行く (go for the purpose of)

The pattern noun (especially activity noun) + に行く ('go to do X / go for X'): 買い物に行く ('go shopping'), 釣りに行く ('go fishing'), 食事に行く ('go for a meal'), 旅行に行く ('go on a trip'). The に here marks purpose, distinct from the destination-marking に or へ.