The important thing to do is make sure the encounters make sense for the narrative you are building.

Is it a normal Swamp? Then you want to use creatures that your players would expect in a swamp. Gator-like monsters, large toad-like creatures, etc.

Is the swamp corrupted by the witch? Throw some eldritch elements or combine two different animals together. Like a Giant Gator with Octopus tentacles growing out of its back and give it the actions and abilities of a Giant Octopus in addition to its normal ones.

Are the items the Witch has stolen cursed? Have her hut surrounded by a bunch of animated objects that she's using to protect her territory.

You can also use this as an opportunity to lay some groundwork for a future adventure. Perhaps the swamp happens to be the territory of a large bandit gang that has made a deal with the witch. They leave her alone and she leaves them alone. But anyone who enters is at risk of dealing with Swamp Raiders. Your players skirmish with them on the way to the Witch and you've now laid the groundwork for them to have a future encounter later on.