These five monsters are a great way to start your campaign and to give your players a fair challenge in the initial levels. I will be only using monsters from the first monster manual of the 3.5 edition of dungeons and dragons.

From Monster Manuel One (3.5)

Bugbear

CR: 2

This monster is a cool monster with potential for interesting  loot. An armour clad, weapon wielding bear creature makes a good level 2 guard and a fair challenge for several level one characters of few level 2 characters. The baddies are chaotic evil making them lore friendly in attacking your party and initiating combat. They can travel in gangs of 2-4 making it plausible for you to up the ante and have multiples attacking your party. (Page 29 Monster Manuel).

Dire wolf

CR: 3

This larger and more aggressive wolf works great for attacks on the trails or in the night when a party has camped out on their way to an objective. The wolf is found in forests and can travel either alone or in a larger pack of 5-8. 1 wolf at a CR 3 would be very difficult for a small party of one but a breeze for a party of level 3 characters. Adding a few of these wolves can help to provide a challenge to a large powerful party and pose an interesting combat. The alignment of these animals is neutral so they most likely would attack for food and if threatened. This monster is simple to describe and easy to implement making it a great monsters to challenge your party with early in your campaign. (Page 65 Monster Manuel).

Ogre (Large Giant)

CR: 3

Like the bugbear, these creatures are easy to implement and fight to their chaotic evil alignment. According to the manual, they travel alone so they are good to pose a fair challenge to a larger party of lower level characters of a sparse few of level 3 or higher characters.They can be used also at a higher level just use the tables to level them up. These creatures due to their flexibility make intersecting repeatedly monsters that become stronger and more threatening as the character level up.(Page 199 Monster Manuel).

Spider Swarm/ Rat Swarm

CR: 1-2

Nothing is scarier than being engulfed or attacked by a large number of spiders or coming across a large number of rats. These creatures are through not too imposing at only a CR1 the swarm attack and defense mechanics and introduce the players to the difficulty of swarms of different creatures. These two creative sets could be used in t conjunction ex. in the same dungeon and fit into pretty much any campaign. Also, you may have a player who can be better suited to dealing with swarms of animals/insects and may be more effective and feel more useful in this type of encounter. Variety is the spice of life. (Page 237 Monster Manuel).

Regular Animals: 

CR: Varied

Probably the most common animals in the world could give your people trouble as they camp or explore. These more simple foes may be good in getting your players use to combat and adding more realism and variety to your game. These animals can be simply tailored to your campaign setting and there are a lot of just normal animals to choose from. The animals could also be tamed by a skilled player or druid adding more variety than just hitting things with swords and providing different ways to deal with an encounter. (Page 268 Monster Manuel).

Honourable mentions to also Consider

Orc

CR: 1/2

These chaotic evil characters will not hesitate to attack your party  and thus are easy to use and justify them as a villainous encounter. However, if you have a half-orc party member these creatures may not be a good fight as them as mindless baddies is less convincing if your party shares a heritage with them and could possibly speak with them. Due to their low CR you most likely will need multiple of them which can be good as it gives each party member their own fight without having to XP share (each could kill one or 2 easily as they level up). (Page 203 Monster Manuel).

Zombies/Skeleton

CR: varied

These monsters are awesome. However, they require more work to implement and create than the average monster. They can be of so many races, classes, and levels that they could be implemented anytime with a skilled DM leveling and creating them appropriately. I would definitely add these in if you have a cleric or players with undead fighting magic or turning magic as it justifies their choices in character and can add to interesting roleplaying (ie. taking control of your own zombie etc.) (Page 265 Monster Manuel).

Sahugin 

CR: 2

If you are doing an aquatic aspect to your campaign these monsters can be used quite nicely. These lawful evil monsters will have no issues going after your neutral or good party if they travel in their waters and can be a pair, alone or in larger groups. They cannot survive for long out of the water and are dependent on having an aquatic setting.  (Page 217 Monster Manuel).