The Google Home Mini is about 50 dollars and is a really cool and popular gadget. Other cool gadgets that are less than $100 are Bluetooth wireless speakers, which you can find relatively inexpensively. These speakers are cool because you can get great sound by playing music from your phone- no expensive system required!
More low tech gadgets include carved wood iPhone cases, and a ring video doorbell which allows you to see who is at your door. You can also find some cheap gaming gadgets that are impressive.