How do I make Low Fantasy more Fantastic?

It depends on what you mean by "low fantasy". It could simply mean that your story lacks a grand quest to save the world from the Dark Lord that spans the entire continent and multiple novels. Alternatively, it could mean that magic is rare or non-existent in your universe. Either way, there are a few things you can do to make your setting more interesting.

1. Original species, especially (though not necessarily) sentient races. Creating original flora and fauna for your setting can add some extra exoticism without turning up the magic level. Real creatures and combinations thereof can provide inspiration.

2. Unique and interesting cultures. Try coming up with exotic cultures with languages, modes of dress, cuisine, laws, etc. Detail their history and how they interact. This will immerse a reader in the setting.

3. Non-magical substances with curious properties. For example, a type of wood that is both strong and lightweight, making it ideal for use in airships. If these substances are hard to come by, that could set up a good conflict between those who need or want it.

4. Religion, folklore and superstition. Just because magic isn't real in your setting doesn't mean people cannot think it is. There could be stories of divine miracles and fey spirits that roam the woods.

5. Legends of great persons and great deeds. Your protagonists may have grown up on stories of great heroes, dastardly villains, and mighty kings. They can provide inspiration for those living in the present, and maybe one of them left some treasure lying around.

These are just a few things you can add to make your setting more fantastic. The details are up to you, and will depend on what you want out of your constructed universe.

 

In general, if strange and arbitrary things happen without much discription or explanaition, it gives a fantastic feel.
When you have a name or explanation for something, it becomes something you can accept and deal with, but as long as you don't, it remains outside of your normal, and therefore fantastic, or s
A giant creature with a house on its back, enourmous teeth like spears, and a tentacle on its head is far more fantastic than an elephant. If a blast of fire appears, it is more fantastic then a Fireball No. 11, class B. The more room you leave for readers to do worldbuilding in their heads, the more fantasy your story will be. This is why Niel Gaiman's low fantasy oozes mystique and feels like fantasy through and through, while Brandon Sanderson's High Fantasy feels like Sci Fi.

 

Strange flora and fauna is the way I would go also. If it were me, I would create an ecosystem for the various places my character would visit and make it part of the story. How they come these natural obstacles.

 

Depending on the word the fantasy elements don't need to be up front and present. There can be legends, or stories, or personal experiences that add to the notion of fantasy/fantastic/super natural elements within the world. This may nudge it the direction you're shooting fore, but as was said, if you need more fantasy in the plot, then add more fantasy.