Right from the get-go, you can see and item in a little hollow by a Pokémon Breeder. Inside is an Ultra Ball, but good luck getting past without being seen by the Breeder with his Tranquill and Liepard. Ahead are a couple of Pokémon Rangers who give berries for beating them (Cheri for defeating Forrest and Pecha for defeating Serenity.) There is also a Net Ball lying out in the northeastern part of the forest, which is good if you want to catch a Bug type (or Water type later.) Also in the northeast part is a Hidden Grotto, which contains Combee, Leavanny, Pinsir, and Heracross. The latter two are almost version exclusive, with Pinsir being more common in Black 2, and Heracross being more common in White 2.
To the west is a hidden Leaf Stone in a dead end, and a Hiker who talks about a Zoroark who lived in this forest previously. He also gives you TM 95: Snarl. Snarl is a 55 Damage, 95% accurate Dark type Special move that does damage and also lowers the opponent's Sp. Atk stat. It targets both opponents when used in a Double Battle, so it is quite handy if you plan on doing the PWT later in the game. The hiker then turns into a Zoroark and runs off.
At this point, there is not much left available to you, as you need Surf and Waterfall to reach further into the forest, which you do not have at this point in the game.
New Pokémon in here include the Elemental Monkeys, Pansage, Pansear, and Panpour. All three of these monkeys have the same stats and abilities, but different types and movepools. All three of them are mixed attackers, and have a base 101 Speed, which outspeeds quite a number of Pokémon. They also all have access to Acrobatics, which they all get benefit from (Pansear, however, mainly uses it for output more than taking care of a weakness.) There is also the aforementioned Bug types, all of which are useful for Physical attacking when evolved, with Heracross being the best because of Guts. Be wary that you pick up a female Combee, however, as male Combee do not evolve into the mighty Vespiquen.
Lastly, there is Roselia. Roselia's family is a Sp. Attacking force that is to be reckoned with. Roserade possesses a 125 Sp. Attack stat, a 90 Speed stat, and a 105 Sp. Defense stat, so it can take a few resisted hits and dish out damage. The biggest flaw, however, is that some of its good moves are given by breeding, and the 55 Defense is meddling. It is definitely something you may want to consider for a team.