You've liberated Saffron City from Team Rocket's mad scheme, and you could probably use a bit of a holiday. We hear the fishing is good down south of Lavender Town, so grab your best rod and get set to go!
You ought to pack the Poké Flute, too. You won't be able to avoid Pokémon duels completely on this trip, but things will be far less hectic. Route 13 is more like a maze than a highway, but it is a pleasant place to walk and enjoy the great outdoors.
Route 12
Sleeping Beauty
The Poké Flute is needed to wake the Snorlax blocking the path here. But first, visit the gate's second floor to get TM27. It contains Return (which, the stronger the friendship level of your Pokémon, the more powerful it is).
Snorlax won't be very happy to be woken, but try to capture it. This is the only other Snorlax in the game, so if you didn't catch the one in Route 16, here's your last opportunity. With the way clear, continue south to Route 13. After the Snorlax, get the Super Rod from a man in a house.
TM27 Return (Given to you by the girl on the second floor of Lavender Town's gate)
Hyper Potion (Hidden near a fisherman)
TM48 Skill Swap (On a small island at the North of Route 12, through surfing)
Leftovers (Hidden and "buried" on the spot where Snorlax stands, unearthed with the ItemFinder)
Super Rod (Given to you by the Fishing Guru at a house south of Snorlax's spot)
Net Ball (one or more, up to a certain limit) (Given to you by the Fishing Guru for showing them a Magikarp and then, if possible, anytime for showing them a bigger Magikarp than the ones previously shown, theoretically limited to certain minimum and maximum sizes)
Iron (In the grass area accessible only by cutting a tree, south of the Fishing Guru's house)
Rare Candy (Hidden in the patch of grass on Route 12 (three grass patches up from the bottom and two to the right from the left)
Leftovers is an important and popular item to be held by a Pokémon. It recovers a Pokémon's health one bit at a time, making it universally useful to any Pokémon.
PP UP (Hidden in the gap in the southernmost fencing, next to a strange Trainer Tips sign that tells readers to look, referring to this item)
New Pokémon
#132 Ditto
Ditto may have the ability to Transform into any Pokémon it faces, but with such weak stats, it's almost not worth the catch. However, Ditto does have one factor that makes it worth it. Because of its Ditto egg group, it can breed with any egg group except for its own and the No Egg egg group. Find one with some good IVs, and you'll have an excellent breeding Pokémon on your hands.
#98 Krabby
Krabby is a fairly good Water Pokémon, and you can have fun beating things instantly (30% of the time) with his Guillotine technique. But Krabby's real selling point is that he can learn the Cut, Surf, and Strength HM's, allowing you to have them all without using more than one slot on your bench.
#116 Horsea
Horsea's evolved form Seadra is a solid fighter, and learns a good variety of Water-type attacks. But you probably won't want more than one or two Water-types on your bench, and there's a variety of better options. Cloyster, Vaporeon, Starmie and Lapras top the list.
#72 Tentacool
Tentacool mixes Water with Poison types, and learns a variety of good attacks in both (and has a high enough special to make good use of them). The Built-in Poison attack Acid does damage while lowering defense, which is nice, but Poison types still aren't stellar.