In general when using Gnome I try and avoid extensions. Most of the 'default' setup is fine or I was able to get used to it and it works well enough. There are a few extensions I do use however for various reasons. All of these work with Wayland.

• AlternateTab - I use alt-tab a LOT. I just found myself unable to get used to the default behavior of showing applications and windows under those, when I use alt-tab I want to switch to a specific window quickly, not have to switch to a application and arrow key around to find the one I want.
• Openweather - This is just fluff, but nice to have. Gives you a nice little weather report in the top bar. You do have to pick a provider and get an API key, but it's pretty painless. You can also setup multiple locations.
• PanelOSD - This one I use to move notifications over to the top right side. This is where I am used to them being from Xfce and that way they are not in my way.
• TopIcons - This allows you to see systray icons on the top bar. There's still a few applications I use from time to time that have a systray and it's handy to see them if they do.

I'd probibly include the redshift extension here, but it doesn't work with Wayland. :(