Perhaps you are right, but I rank the Louisiana Purchase, the Lewis and Clarke Expedition (which I am almost certain he commissioned but not absolutely certain as I am writing this), and the abolition of slave trade as extremely significant.
The Louisiana Purchase was a bold act that he (as a strict constructionist) arguably had no right to do. However, it set the table for manifest destiny that dominated 19th century politics and served as the backdrop of many major accomplishments of future administrations.
The Lewis and Clarke commission was a minor thing for someone to commission (not minor to the Lewis and Clarke crew to endure and accomplish, mind you). Whether he had the forethought to realize it's import for the 19th century might be argued, but is is inarguable that it was an extremely significant accomplishment for the future of this nation. Perhaps it compares very favorably to the race to the moon. The accomplishment itself is amazing, of course, but its execution was not the most significant thing. All the things discovered, invented, enhanced, etc., in putting a man on the moon and the propulsion of these things via the private economy to escort us into and beyond the information age is the enduring legacy of the race to the moon. Likewise, most every significant accomplishment of the 19th century traces its roots and owes tribute to the Lewis and Clarke expedition and the Louisiana Purchase.
The abolition of the slave trade was an important act that set him apart from virtually every other 19th century President until Lincoln. Virtually all of them "kicked the can down the road" on this issue until Lincoln. None that I remember even lifted a finger to use the bully pulpit to even try to move public opinion. And according to some Lincoln would like to have done the same but to his everlasting credit (pure motives or not) he did embrace it and abolished the practice of slavery via the civil war.
Again, I am not a scholar and someone more knowledgeable than me may correct me about the slavery issue (it could be that other Presidents indeed did try to do something to help the slaves) or the Lewis and Clarke Expedition commission date, but if I am right I think these accomplishments significantly shaped the 19th century adn beyond for this nation.