This app was mentioned in 10 comments, with an average of 2.30 upvotes
Have you signed into your google account on the watch? You'll need to do that. There's actually a google play store app on the watch itself. You can install things from there, but the browsing experience on the watch leaves something to be desired. Still, it should let you know whether you can install things successfully or not.
Not all apps support all devices. You will need to find an app that supports watches in order to install it. Here is the first result I found for a calculator app for wear: https://play.google.com/store/apps/details?id=rocketstartups.wearcalculator&hl=en_US
I use this. I haven't found a calculator that would work in ambient mode though. It is really annoying that I need to keep opening it when the watch screen goes to sleep.
The solution I found is that I configured Bubble Cloud to open it in sticky mode, which means the calculator will come back to the foreground when I tilt to wake my watch.
Already got one and I like this app! https://play.google.com/store/apps/details?id=rocketstartups.wearcalculator
/u/Miraclefish has covered most bases, so I'll just recommend Calculator For Android Wear - really handy.
Edit: Oh I also recommend Facer for customising watch faces, and https://facerepo.com for downloading existing custom watch faces made by others, all for free.
Edit 2: Navigation wasn't mentioned either.. basically yes you can start navigation from your watch with a voice command. Map & instructions will be shown on the watch, plus the phone will start speaking the instructions and running full Google Nav throughout. The reverse works too.. if you start nav on your phone, the watch will join in.
Putting a calculator app on it has come in handy for me lots of times. This is the one I've always used. Simple, but handy.
Also, if you have Android 5+, set up your phone to lock automatically when it's not near your watch!
As a don't: Don't be surprised if navigation or music playback (from the watch) devastates your battery. Here are some watch faces I have on mine.
Watch Faces:
I find WearCalculator to be constantly useful
https://play.google.com/store/apps/details?id=rocketstartups.wearcalculator
Calculator for Android Wear very complete, no need for phone.
"Sitting is the new smoking" Wear Stand-up Alert works completely on the watch. All the settings are available on the watch (and only on the watch if you don't also have BCL installed). It is said to lengthen your life, pretty good use for a smartwatch!
Password manager Safe in the Cloud has a fully functional smartwatch component, you can have your passwords on your wrist! They do fingerprint reading (on Samsung phones too) and cloud syncing. I think it's the best password manager.
I put on a calculator for small basic things like in the grocery store.
It's called Calculator for Android Wear
There's also STO Card for reward cards. Has a Android Wear version that shows the barcode. Tried it at a Self-Checkout but didn't have much luck but YMMV.
stoCard
Also a fun silly one like WhoopeeWear
WhoopeeWear
Just a couple things I found. Hope any helps..
Best calc for wear: https://play.google.com/store/apps/details?id=rocketstartups.wearcalculator