(covers information from several alternate timelines)
Monuments did not need to be physical objects, and the term could even be used abstractly. For example, a race of Ancient humanoids engineered humanoids to be a dominant life form as a monument to their existence. Gosheven described the colony on Tau Cygna V as a monument to everyone who had lived there. (TNG: "The Chase", "The Ensigns of Command").
More informally, in 2372 Julian Bashir described the state of Miles O'Brien's quarters as a monument to his year as a bachelor. Similarly, in 2375, Ezri Dax, through her former personality Joran Dax, snarked that the crew quarters of Greta Vanderweg as a monument to bad taste. (DS9: "Accession", "Field of Fire").
Figuratively, the term "monumental" could be used to describe particulary large works, or moments of particular historic significance. On Kataan, the construction of atmospheric condensers was considered a monumental task, and therefore unfeasible. (TNG: "The Inner Light") crewman Daniels described the Battle of Procyon V as a monumental moment in history. (ENT: "Azati Prime")
Gul Darhe'el was buried under one of the largest monuments on Cardassia Prime, a fact which Gul Dukat witnessed and offered as proof that Darhe'el was indeed dead when Aamin Marritza claimed to be him, alive and well. (DS9: "Duet")
In the 31st century, or at least the version of it from which crewman Daniels originated, Earth hosted a monument to the United Federation of Planets. The monument disappeared in a version of this future in which due to the Temporal Cold War the Federation did not exist. (ENT: "Shockwave, Part II")