A river drake in the river, accompanied by 2 tatzlwyrms in the trees and a handful of kobolds. The encounter begins when the PCs are spotted by the kobolds or tatzlwyrms passing near the river. The kobolds use slings from inside the tree line, then flee backwards to the water's edge. The edge of the tree line however is trapped with minor things - snares, nets, whatever, that will deliver Entangled and slow the PCs' ground movement.
The tatzlwyrms drop down on them from above then, attempting to grapple, weaken with poison, and claw the PCs with Rake. If possible, some kobolds might split off from the riverbank to try and pile into one of these grapples, getting out some rope to attempt to tie up a character.
Eventually either the kobolds and tatzlwyrms get one or 2 of the PCs grappled and tied, then try to drag them to the waiting river drake or the characters defeat their attackers and see only a few of them left on the riverbank in the distance and hopefully move up close enough that the river drake can attack them
The final portion of the fight should be the river drake, with at least a few kobold minions, battling the party at the water's edge, using its caustic mucus from distance or its speed surge to get into a melee with a single foe separated from the others.
Remember that the kobolds inside the tree line have either partial cover or cover, and against PCs that lack Darkvision or Low Light Vision they might be able to get Concealment in the darkness. You might also provide squares of overgrowth that the kobolds can use to grant Concealment regardless of the PCs' senses. This can not only mess with ranged attack accuracy but also limit line of sight for spells.
Also remember that fully submerged monsters have full cover and even just popping up its head out of the water to spit mucus gives the river drake cover, in regards to ranged attacks. Hitting the thing while it's in the river should be pretty hard unless the PCs are willing to wade into the water after it.
Having 3 phases of attack, scattering some traps around, putting monsters above using their Climb speed, foes moving up and down the terrain, grappling instead of straight-up bite attacks, and so on could keep things interesting. Another interesting thing might be to have a goal other than pest control.
Perhaps the kobolds are worshipping a dragon in the area. They believe that the tatzlwyrms and river drake are divine beings and have built them a shrine because of it. As the PCs are approaching, perhaps they spot signs of draconic worship in the area; fetishes hanging in the trees, a way marker sculpted of stone, depicting a dragon's face, or scrolls invoking the power of dragons hung from vines as a sort of "keep out" message.
Once the fights make it down to the river's edge, the PCs might spot mud structures in the distance. Somewhere, in the midst of the river, is a shrine to their dragon god. The ambush is meant to defend the shrine from interlopers. During the fight the kobolds bark that their cruel god will curse these trespassers for their insolence and such.
Then, when the fight is done, if the PCs are victorious and search around the mud hut village of the kobolds, they find pieces of a collapsible bridge that spans the river to the shrine. You might put a curse on the shrine for non-kobolds or non-dragons entering it, or maybe not depending on how involved you want to be with this. Even from the shoreline when the characters are searching the mud huts they should be able to see suggestions of wealth in the shrine, encouraging them to go there.
The shrine is the final reward at the end of the encounter, but it might also hold a terrible secret for the characters. You might, even have the river drake here instead of earlier in the scene. Any way you slice it, raiding the shrine ends the whole thing for better or for worse. If you DO put a curse on the shrine for any PCs entering, it might not just be something to weaken a stat.
Interesting curses might be that any gems handled by the cursed PCs are suddenly discovered to be flawed and worthless; jewelry turns out to be glass and costume jewelry; all coins turn to copper pieces. Perhaps, in an even more draconic twist, any PC that fails to save against the curse becomes insatiably greedy, desiring all the loot for their own "hoard." Now any PCs not affected by the curse have to restrain their fellows and find a way to not get robbed in their sleep until the curse can be lifted.
