Wait, HPMOR was lighthearted? That's not quite how I'd describe it. But in the same vein, try Worm [0]: A take on superpowers that tries to include changes to the world it's set in. Highly recommended by the author of HPMOR - "The characters in Worm use their powers so intelligently I didn’t even notice until something like the 10th volume that the alleged geniuses were behaving like actual geniuses". Also now has a sequel, Ward [1]. But be warned - "people who get powers continuing to act like people" does not a happy world make. (Also, it's really remarkably long. Like, current length of the ASOIAF series long.) Goodreads: [2]

Following a slightly more conventional definition of lighthearted, I think I'd recommend Larry Niven's Ringworld. When it comes to "moderately plausible megastructures", the ringworld concept is by far my favorite. Much less complicated than a Dyson Swarm, after all! Story is ok, and the sequels expanded things well enough. Not exactly the biggest fan of his female characters. Still worth it. Triple Crown (Hugo/Nebula/Locus) winner. Goodreads: [3]

0: https://parahumans.wordpress.com/ 1: https://www.parahumans.net/about/ 2: https://www.goodreads.com/book/show/18713259-worm 3: https://www.goodreads.com/book/show/61179.Ringworld