What's a good gift for a 2-year old?

Just ask the parents what (s)he likes.

Definitely coordinate with the parents, not only to find what your nephew's interests are but also so no gifts are duplicated.

If they don't have advice or you can't ask for some reason, Brio scale trains, construction equipment, emergency vehicles, dinosaurs and generic toys are usually a good bet, they don't usually understand or care about brand names or franchises yet.
- if they don't have some already, 100% on the Brio train set - except not Brio. The generic wooden railway sets are just as good nowadays, and many many times cheaper.
  
  You can buy a whole dinosaur themed wooden railway set on Amazon for £37. I'd want that, even as an adult!
  
  FWIW, both of my kids played with their wooden railway for years - seriously good investment.
"They" is a word that exists
- Full stops also exist. Both are fine.

Serious.

World map for their room. First teaches colors and shapes, then numbers and letters, and then history.

Are there world map rugs, kinda like the classic "street map" children's rug? I feel like that would last longer.
- https://groovebags.com/products/blue-world-map-area-rug
  
  I did a search.

Nothing that makes noise or takes batteries.

Board books. Toddlers are murder on books with paper pages. Stick with classics like Goodnight Moon or Dr. Seuss, or if you want something more recent the Little Owl series by Divya Srinivasan, or Calm-Down Time by Elizabeth Verdick are really good.

Playground/beach toys, like a bucket / shovel / rake set that parents can keep in the car. They'll likely get lost, so go for cheap and sturdy over premium and expensive.

Sidewalk chalk, like the huge fat ones.

The water based coloring books someone else mentioned are great, too. No cleanup (beyond the occasional water spill), they can be reused, and they're great entertainment for the car.

And yeah, like others have said, coordinate with the parents.

And if you hate the parents, get a Furby knockoff or something else that makes noise.

Get the pop up book adaptation of Stephen King's "The Girl of who loved Tom Gordon"
- The Pop-up Book of Phobias is also great.

Ask the parents what they need.

For the kid themselves just bring soap and a bubble maker, enthusiasm, and a big smile.

Get them a magic painting book.

They are black and white illustrations that have the ink in the black parts. The kid takes a brush with some water and scribbles to create color. It's just perfect for that age, and it's not more junk that the parents need to collect.

Source: have toddler under two and friends with babies that just turned 2.

Yes, these are great for that age!

If you hate the parents, something that makes noise

Lego Duplo? There are some very lovely sets out there, my husbands nephew really enjoyed the little animal train we got him :)

If you hate the parents? Something loud.

Otherwise ask the parents.

At two they are growing and changing quickly, clothes are good too

A toy that makes a lot of sound- like a truck with siren noises or a little electric piano, or spaceship with explosion sounds. There are these great plastic dinos that roar when you play with them. As a parent, I’m telling you, their parents will thank you.

Every kid is different, but if you're not able to ask the parents then a safe bet is a duplo set. Bristle blocks are also great. My kid is currently 2 1/2 so these are what I know would be hits with him and his friends.

You could also get one of the following books:

Goodnight Goodnight Construction Site
Don't Worry Little Crab
The Little Blue Truck (it's a whole series, the Halloween one would be appropriate)
Bear Snores On or any other book in that series.

Seconding Don't Worry Little Crab. It's great.

A nice book with lots of pictures.

Magnetiles are probably a good choice. Gonna get my niece a set for her birthday next month.

A large empty box that they can climb in and make it a fort. Cover it in cloth with their favorite theme.

A gun

A cardboard box (and put something in it so the parents won't look at you funny)

Wrapped in the shiniest paper!

Teddy bear

Stuffed elephant.

triple shots of espresso

A quiet book is pretty age-appropriate for a toddler.

Children's books.

Iirc, we introduced a strider-style bike around that age. Even ride-on toys can fill that role as we in the northern hemisphere move into the colder months.

give the parents a date night. give the kid some dinosaurs.

Fire truck!

I have a 2.5 year old girl. She loves stuff like this. This brand is also pretty durable.

Play dough is a good one.

Make sure you get nothing with battery’s and/or makes noise unless you hate the parents

This little people house.

https://www.walmart.com/ip/303615936

It's kept my daughter interested and her several years brother loves to play with it too. It's got a few cute songs that she dances to. There's one about mixing milk and spaghetti and some nonsense. It was well worth the money. As a kid, I remember playing with the little people house up until like 10 or something so it doesn't get old.