Grandpa Lou's one scene in the movie has him watch Nigel Thornberry's show with Tommy on his knee while warmly telling his grandson about how Nigel is brave and finds his way out of every dangerous situation. Even if it's one brief scene, it's nice that Lou's short appearance shows him bonding with Tommy as usual.
After Stu's boat is destroyed and the group is sitting forlornly on a raft, Angelica of all people starts singing a hopeful song in an attempt to lift their spirits. Even better, it's immediately followed by Spike (who seemingly died when the boat was destroyed) swimming up to the raft happily...before knocking Cynthia off of the raft.
When Chuckie sees a baby loris (Chuckie assumes it's a monkey because it kind of looks like one) being hassled by two larger ones that want the mango it's holding, he quickly stands up to them, telling them to leave the little guy alone which actually prompts them to flee. The baby loris then smiles and gives Chuckie the mango out of gratitude.
When Nigel Thornberry returns to his normal personality after getting conked on the head in the bathysphere, his first response to seeing the Rugrats gang is to gush over how adorable they are. He is also quite happy with observing the elusive giant squid with the children.
Spike's entire demeanor toward the babies when we get to find out how he talks—he specifically refers to the babies as his babies, to the point that Eliza assumes he's talking about puppies. When Siri enthusiastically asks if he's referring to "helpless offspring", Spike immediately says "yes". When given the chance to save them by retrieving the raft to patch up the comvee's pontoon, he willingly risks his life to save them, knowing full well that there's a chance that he won't survive. Spike sees the babies as HIS children and is fully willing to die for the sake of their well-being.
When he's submerged in the water and losing consciousness, Spike starts to hear Stu and Didi's voices praising him and Tommy and Dil's giggling in his head...which prompts him to swim as fast as he can toward the raft with newfound vigor.