My favorites are:
Alexa says "The microwave has finished", 1 minute after the power draw from the microwave drops below 50 watts. The message repeats 5 times (my wife sometimes ignores all 5 of them). 
Once the door is opened for at least 2 seconds, the rule exits and it stops repeating.
Alexa says "Please close the microwave door" 1 minute after it has been left open and the microwave power is below 50 watts.
I accomplished these by using a contact sensor and one clamp from an HEM to measure the power on the circuit the microwave is plugged into. It could be accomplished with an energy monitoring plug like the Zooz ZEN 15 too. If you just attach a contact sensor to the door, then things are simpler than I have setup. My microwave is above a wall oven with a frame, so a contact sensor on the door wasn't going to work. I was too chicken to tap directly into the controller board, so I connected a relay to the microwave light. The relay closes and those contact are attached to the hall sensor of a SmartThings Multi-sensor, but any compatible contact sensor will work too. This way of doing it adds a level of complication since the light is also on when the microwave is running, so I have to measure the microwave power at 1 second intervals, and that causes a world of pain if you don't have that one a hub by itself.
So while this is a bit complicated, it's a really cool feature that I can adapt to ANY microwave oven.
Another favorite is Alexa says, "The washer has finished, and "The dryer has finished". These are accomplished differently for me since my dryer is old and my washer is a high efficiency front loader.
For the dryer, I connected a relay to the start button. In my dryer, when you push the start button, it physically connects contacts that both start the dryer and engage an electromagnet that keeps the contacts connected. When the timer is finished, power is shut off to the electromagnet, thus the dryer stops. By connecting a relay to the electromagnet, I get contacts that have no voltage on them, and are connected to another SmartThings multi-sensor in the exact same way the relay on my microwave is.
For the washer, it's more complicated, because my washer is so efficient that the running power is often the same as the idle power. I use the other clamp of my HEM to measure the running power, and compensate for pauses. However, while this works for most, it was inconsistent for me. So I also had to add a Homeseer Flex Sensor. This can tell the difference between the running light on my washer being solid, flashing, or off. So I get a trigger in my rule when the light has gone from solid to off for an extended period of time. But my washer being the pain in the arse it is, will give a false trigger if you pause the machine to add some laundry. So I measure BOTH the light status AND the power. This is 100% accurate (and it took me a long time to get to that point). The flex sensor is the best device for most washers in my opinion. My washer is also 220v, so that meant that the most economical way to do this was to use one of the clamps from an HEM.
The great thing about knowing when they are running, and not just when they are finished, is I have setup a routine in Alexa and tied in virtual switches, so I can say "Alexa, laundry status" and it will tell me if the washer is running, the dryer is running, both are running, or neither is running. We also get HE notifications when either machine has finished.
One very simple lighting automation that I do really like is to have my kitchen light turn off when it's after 11:30pm and no motion has been detected in the kitchen for 10 minutes. This behavior ends at sunrise.
That's it. Not complicated, but I do love to see the kitchen lights turn off by themselves when I see it in action!
A very useful automation is to again use a contact sensor for door open notifications, but on my fridge. Those of you with newer fridges that already have this are yawning right now, but my fridge is old and I don't have that. It's a top freezer type, so a single contact sensors covers both. I use a Xiaomi Mijia sensor because they are nice and small, so it's not this big ugly thing on the door. The sensor part is on the fridge door, and the magnet is on the freezer door. It took time to get the alignment just right, but if either is just slightly opened, like happens sometimes if something isn't put away properly, it triggers.
I have Alexa say "Please close the refrigerator door" after 1 minute, and cancel when closed. Simple but very effective. Once I put this in, I found that it triggers a lot, and that means I'm saving a lot of money with this simple automation!
Last that you might be interested in is the auto unlock of my front door. This isn't for everyone, but I've got mine working very reliably. I use three triggers to avoid any false activations. When either my wife or I leave our geofenced area with our phones and then return to the geofence, the first trigger is set. When either of our phones connect to our house WiFi, the second trigger is set. When a motion sensor at my front door is triggered and the other two triggers are also active, the door unlocks and our alarm system disarms.
The phone geofence and WiFi tiggers are then disabled until we leave the geofenced area. This prevents them from unlocking the door if a someone other that us walks up to the front door. The triggers are unique for my wife's phone and mine. So it must be one of us entering the geofence, and it must be one of our phones connecting to the WiFi.
but I couldn't see how to do it in fewer rules, and for some people, that just drives them nuts. For me, meh. If it works the way I want, I'm good.
) goes completely dead and then she charges it when we're home. What was happening is as soon as the phone turned on again, it was changing the HomeKit presence as if she had just re-entered the geofence, and if someone happened to be standing at the front door when that happened, since her phone was also on the WiFi, it would unlock the door when we didn't want it to.
