Don't close the shiny holes, bury 10k in them instead. As for decorating, I recommend planning it ahead on
Happy Island Designer. Do keep in mind any cliffs need stable ground in all directions, just like planting a tree. Also, if you have a 2x2 square of cliffs, you can only round two corners in a diagonal.
Resident services (the plaza) can't be moved, and neither can the pier (tiny strip of wood out in the sea) or river mouths. Most people like having resident services lined up with the airport, so if that's something you'd care about, feel free to save strat when you get to the island selection screen.
Save your nook miles for Nook Mile Tickets! Only spend them on things you've researched and decided were worth it: hairstyles/colors you actually will use, your loan, and inventory/tool enhancements.
Use your Nook Miles Tickets at night. There are no islands that are only useful during the day, and two that are REALLY profitable during the night.
In ACNL, balloons were pretty much useless, or at the very least safe to ignore. That is not the case with ACNH. Pop down every single balloon you hear.
You have to expand your house A LOT to finally get the option to relocate it. Put it somewhere you won't regret in the next week or so.