Normally, ie, prior to this current virus crisis, the Military Museum in Paris is not crowded. I've been there at that time end of June/early July over the years.
The most recent visit was in 2018 when the Museum held a special exhibit focusing on "Napoleon as Strategist." Very elaborate using the battles of Austerlitz and Waterloo, his greatest victory, where the armies of two Great Powers arrayed against Napoleon were destroyed in a single day, the first time in modern European history, and Napoleon's decisive and final defeat at Waterloo, requiring the combined armies of Prussia and England to overwhelm him.
I saw the Pantheon once in 1999, also in July. It was not crowdeed. Like The Military Museum who knows with virus crisis?
You might be lucky in seeing a miltary "band" performing in the court yard of the Les Invalides. I saw that once, (2003?) basically lucked out seeing the musicians dressed up in the uniforms of the Old Guard of Napoleon's Imperial Guard playing the French military marches of the era.