A futuristic city? Maybe you can incorporate a subtle cyberpunk look into your island. There’s plenty of videos on cyberpunk island tours. An industrial look could work as well…
Otherwise, I found these on Google.
The last two photos came from an island tour posted on Tumblr. Here it is:
LINK
I don’t know if you plan to use your house as part of the set-up, but if you do here are some ideas.
—Space cafeteria
—Control center (located on earth or space)
—Training facility (Don’t astronauts train in a swimming pool sometimes? Idk)
—Lounge area (Artsy chair and table could fit in here)
—Space themed arcade
You can also use gyroids and (fake) ancient statue as tiny aliens.
As for items, definitely get the Celeste diys. Maybe the rock diys as well. The orderable items shouldn’t be too hard to get if you have the dlc.
Items with “space” tag
-Futuristic meal
-Science pod.
-Large display case (if you want to create minimalistic, white buildings)