I dig the idea of having a few baddies in the area manipulating the bandits to otherwise occupy the heroes while the baddies orchestrate their master plan. Maybe the bandits are just the distraction their evil arch nemesis realizes would occupy the heroes? Perhaps the PCs, in dealing with the bandits are able to discover a trail of clues leading them to the baddies?
Maybe the bandits are the surface arem of a bunch of Drow hell bent on exacting revenge from the Demonweb Pits Debacle?
Alternately, what if they aren't just bandits, but in fact some sort of new mind flayer sub-type -- a mind flayer wearing a bandit skin? Might give the players a rather nasty challenge.
Maybe the Bandits are actually storm giants cursed with a reduce person effect bringing them down to normal person size? You said this was forgotten realms, maybe Elminster is up to his usual shenannigans?
Anyway you cut it, the possibilities here are prety limitless :)
At this level, the bandits as written are never going to be a threat in a straight-up combat with the PCs. Guerrilla tactics will work for a bit, IF they can manage to slip away from the party after making their initial attacks, but chances are your cavalier will be able to spot them and chase them down, and the wizard will be able to scry-and-fry on them if they get away. At best, they're going to be annoyances and maybe use up some of the party's resources between major encounters.
Since you don't want to level them up, you're going to need to give them some solid backup if you don't want to take Jabber's suggestion of "make them something more than bandits". I'm going to assume that his suggestions come out in the end to basically equivalent to "level up the badguys" and thus not repeat the ideas.
Since you said high-level encounters aren't an issue, I won't bother telling you where to find appropriate challenges. But unless you get the party stuck in a "Tucker's Kobolds" situation where the bandits are really, really prepared for them (which hey, if you can pull it off, more power to you!) there's just about zilch they can do to keep the party from steamrolling over them unless you level them up. The gap is just too big.
Jeremias: can't resist the temptation, so I hereby give you:
The Link To True Vaasa - The City In The Frozen North Also Known As Finland
@hargoyle
I know about it... When I searched for nice pictures for my players, I took a few pics of the countryside around the real Vaasa because it fits sooo good to the faerunian Vaasa.
..really? Well, Vaasa is pretty up north in Faerun also so guess that makes sense.
As a sidenote: I've taken a few pictures of them old castles whilst visiting Ireland because they fit like a glove into any fantasy rpg setting. Never thought of photographing my actual surroundings. Mind you, I'm not from Vaasa per se, but your comment proves the old fact that one can become quite blind to ones surroundings..
And on the real topic: I side with Orthos on guerilla tactics. Using them to harass the players settlements could work - although the level gap is quite big.
What are the bandits motives? Money? Independence? Something else? Be that whatever, I suggest you look into some real world guerilla tactics. If the bandits know their surroundins, are from the region etc, they can put up a nasty fight. Like vietnamese vs. american troops in the Vietnam war. This can turn up especially nasty if the bandits are really fighting for some sort of freedom and not just swag.
If you want to make it really real world like, I suggest the old "hidden pit filled with excrement coated spikes" -trap. Imagine your paladin falling into that. Nasty, but not necessarly very fantasy-gamey.
I'd just go with letting your players waste the camps. At level 16 you're in Wuxia/superhero territory, so indulge. Spells that leave scores of blackened corpses, etc.
Or bump the "bandit camp" to Pitax level...