The food fight scene. Only the Robinsons can turn dinnertime into an epic, hammy battle.
The dinosaur fight, full stop. The whole family takes it into their own hands to protect one little kid from being devoured by a freaking T-Rex. By the end of the fight, they may be bruised like holy hell, but they pulled it off - and get a brand new pet T-Rex in the bargain!
Lewis does pretty well for himself too which makes sense, because he will be a Robinson. He executes a perfect jump-and-grab to save Wilbur from being eaten by the T-Rex.
The T-Rex itself puts up a good showing, handling an oncoming train and using parkour to bring down the flying saucer protecting Lewis.
Lewis' Midair Repair of the Time Machine, just in time to prevent him from falling to his death in it.
In the horrible future where mankind is ruled by evil bowler hats, Lewis is literally being attacked from all sides by both said bowler hats and the hat-controlled Robinson family he has come to love. This would be a Despair Event Horizon for many people, but Lewis manages to escape them all and prevent this future from coming about by fixing the time machine he has so far failed to, while flying it. If that's not heroic I don't know what is.
Lewis uttering the Ironic Echo just before he returns to the present to prevent the bad future from happening, and weaponizing a Retcon against Doris.
Just the Robinson Family. They are just awesome.
How about how Lewis defeated Doris? She's wielding her swirling arms with sharp blades and goes in to kill him. He was surprised at first, then he smiles and gives the most Badass words in the most confident voice to Doris: "I am NEVER going to invent you", erasing her from history.