Use the ATmega328P's 8-bit timer and prescaler to implement custom timed events, rather than relying on millis() or delay().
E32 LoRa (long range) device very cheap and very long range (from 3Km to 8Km), here an Arduino shield for my library.
Get your boards fabricated and components soldered using JLCPCB assembly service.
Meet Walbi, a 2:1 scale humanoid with Arduino programs for motion capture and playback. Move it by hand, record and then play it back.
An infinity mirror with an RGB LED strip that can change its color using Infineon's 3D magnetic sensor knob connected to an Arduino MKR1000.
Entertain humans and pets alike with paper planes launched by your voice.
The Water Leak Detector can monitor two specific locations for the presence of water and notify you via a local alarm or smartphone app.