Analogous to Route 5, Route 6 is the connection between Saffron City and Vermillion City. Towards the top of the route (the Underground Tunnel exit), there is a gate and a Rare Candy to the right. It's hidden, so you'll have to do a bit of sleuthing around, but it's worth it. Like on Route 5 (again), Saffron City is inaccessible because the guard is thirsty and needs Fresh Water (Red, Blue and Yellow; any of the three drinks works, but Fresh Water is the cheapest) or Tea (FireRed and LeafGreen). Going south will land you in an area with several patches of grass and lots of trainers, though most of them avoidable; some will look at you when you try and run past, so remember to walk if you want to avoid them (although you should be fighting all trainers for the levels).
Camper Jeff: Spearow (Lv16) and Raticate (Lv16). That Raticate's packing Hyper Fang, so watch out. (the last trainer before Vermilion City, not avoidable)
Red: Pidgey (35%), Oddish (40%), Mankey (25%)
Blue: Pidgey (35%), Meowth (25%), Bellsprout (40%)
Yellow: Pidgey (45%), Pidgeotto (5%), Rattata (25%), Jigglypuff (10%), Abra(15%)
FireRed: Pidgey (40%), Meowth (35%), Oddish(25%)
LeafGreen: Pidgey (40%), Meowth (35%), Bellsprout (25%)