Routes 45 and 46 are built into a mountain, so you will be traveling downhill. You can't walk back up, but you can fly. There are many good items scattered along the routes. Make a few trips to pick them up.
Route 45
Route 45
After completeing the events in Template:~, you should have gotten a call from Professor Elm summoning you back to New Bark. You can Fly, but you might wish to walk instead, since the direct path of Routes 45 and 46 give you the chance to capture two new Pokémon, and revisit the Template:~.
Talk to Hiker Parry
Take the far-left path down Route 45 to talk to Parry. For some reason, this guy is following the Pokémon situation way over in Template:~. Trade phone numbers with him, and he'll give you a call when the Marill Pokémon are swarming over there. You can take the other paths to face the other Trainers and pick up items by flying back up to Blackthorn City, and then taking different paths back down.
MysteryBerry - near pond at the bottom of the route
PP Up (hidden) - surf to the middle of the south pond
Grab some items from the Dark Cave
With Flash, Surf and Rock Smash, you can now fully explore the Template:~ which you last visited when you passed through Template:~. If you enter the cave from Route 45, you'll be able to catch Wobbuffet and Teddiursa, and pick up two cool items: the Black Glasses that strengthen Dark-type moves, and TM 13 (Snore).
Items:
(Route 45 side)
Black Glasses - man in the northwest corner
TM 13 (Snore) - near the entrance to the other side
Revive - southeast corner
(Route 31 and 46 side)
Elixir (hidden) - dead-end in the northeast
Dire Hit (Crystal only) - northwest of Route 46 exit
Hyper Potion - northeast corner north of Route 46 exit
Full Heal - north of Route 46 exit
Route 46
Route 46
Note that the other end of the Dark Cave is here, but you cannot reach it from the Route 29 end, only from the Blackthorn City and route 31 after surf and rock smash (crystal) have been learned.
Not much has changed with Jigglypuff itself (although many of its old moves have been powered up a bit in Gold/Silver/Crystal). But like Pikachu, it can now lay an egg containing a new Pokémon, Igglybuff.
Gligar is Ground/Flying, possibly the most bizarre type combination in the game. It's nice to have a Flying-type that's immune to Electric, but Gligar's moves are weak and it can't even learn Fly.
#227 Skarmory
#231 Phanpy
Many combinations of Flying type and another Pokémon type don't always work out too well. But Steel/Flying works for Skarmory, since it makes it weak to nothing but Electric and Fire. Even better, Skarmory can learn Fly, does learn a Steel technique, and has a great Defense score.
Phanpy learns moves slowly and isn't anything special by itself, but its evolved form Donphan is a powerful fighter with HP, Attack and Defense stats that are through the roof. Note that if you evolve Phanpy at level 25, it will learn Fury Attack. If you don't, it will learn Take Down instead. Take Down is probably the better move, but do evolve it into Donphan shortly thereafter, since Phanpy can't learn Earthquake.
Geodude
Graveler
Gligar
Teddiursa
Phanpy
Donphan
Skarmory
Spearow
Rattata
Jigglypuff
Magikarp
Dratini
Dragonair