Bluweil Forest
"Well, it's always safer to travel by road and by day, obviously. But taking risk means you're more likely to find new Pokémons to recruit. Of course, the real question should be what gym you'll take on next. You're still at first tier gyms, right?
That means you can't try to raid the castle, or take on the naval or aerial battles. You'll need to challenge either Vinmur, Boonwagen, or Roarac. Vinmur's the closest, if you're willing to cross the forest again, and Boonwagen's the farthest."
OOC: If you've forgotten, the map is in the Game Map tab. And now is a good time to grill a local trainer on the various challenges!