There are a great number of guardian monsters to help spice up the role playing. Guardian-type monsters are a regular feature in dramatic fantasy fiction, so even new players will relate to plot hooks predicated on guardian monster intervention.
Lord of the Rings, for example, is essentially a guardian-monster-initiated adventure (Gandolf being the guardian monster). The Gandolf example is apt, as many guardian monsters do have polymorph powers, so could assume the likeness of mankind.
For low-level campaigns, dealing with the local dangers of the heath and forest, some of the best guardian monsters you can use are Grims and Moondogs. The great thing about the Grims and Moondogs is that they are elusive and mysterious, so the players would probably think they were part of the evil at first.
Grims take the form of black dogs, black cats, or black owls. The Harry Potter series used the "Grim plot hook" to great effect in book 3, when Harry kept seeing a large black canine stalking him in the shadows (which ended up being his guardian Sirius Black, of course).
Guardians of Specific Places
Some guardians are encamped in a specific locale, working to disrupt and contain some evil from breaking out.
Naga (guardian-types) would be well-used here. From their description it seems clear they would not be likely to be "roaming" guardians. I could see them patrolling a Yuan-Ti infested jungle, or the area around an evil temple...
Foo-Dogs and Foo-Lions are also described as guardians with powerful anti-evil powers. They are described as chaotic, so would not make good long-term disciplined guardians, but they could be brought in as excellent shock troops in the battle against evil.
Opinicus also appear to be well-suited to a role as guardians of some specific location. They are especially equipped to deal with undead and demons, so they would be prone to be found guarding the approaches to dangerous areas full of such creatures, such as the forest around a vampire's castle or the mountains around lich's lair, for example.
Wandering Guardian Monsters
The Agathion, Baku, Ki-rin, Lammasu, Hollyphant, and Shedu are the classic wandering guardian monsters. They are all described as "roaming" guardians, seeking out and containing evil wherever they can find it.
Most of them can polymorph into human form. I imagine they would maintain some kind of contact with locals on the ground, gathering their information. Naturally, such contact would tend to be with influential druids, clerics, magic users, or psionic warriors.
Dragon Guardians and Dragon Wars
Another creative idea would be to use a dragon, or even Bahamut the Platinum Dragon, as a guardian. His description clearly describes his regular wandering. No doubt he is gathering intelligence and keeping tabs on rising evil, ready to intervene if necessary. I would imagine he would be especially vigilant against dragon problems, particularly related to his evil arch-nemesis Tiamat.
I can imagine lots of plot hooks involving a "War Among the Dragons". Imagine the poor people caught in the middle of those battles. Of course, the good dragons would seek out the help of powerful humans of good alignment to assist in their battle. The evil dragons would perhaps have some evil humans on their side, and let us not forget the Githyanki-Red Dragon alliance...
You gotta admit, campaigning a Dragon War would be pretty freakin epic.
Good old-fashioned Guardian Angels
The ultimate guardian monsters are probably the "guardian angels" statted out in the 1e MM2 as Devas, Planetars, and Solars. If you are doing a "human-centric" campaign, these would be perfect. Perhaps especially if you are doing a "war of good vs evil, angels vs devils" type scenario. Solars are so ridiculously over-powered, they would have to be used against Demon Lords and such.
Guardian Monster Random Table
Or maybe you just need a guardian animal to bail your players out of a tight spot or severe challenge. Here you go, in random tabular form:
31-38 Naga, guardian
39-46 Foo-creature (75% dog, 25% lion)
82-88 Lammasu (25% Greater Lammasu)
89-95 Shedu (25% Greater Shedu)
96-97 Dragon (10% Bahamut)
00 Planetar (25% Solar)