If you want a smart smoke detector without removing the devices already installed at your house, just get the Roost Smart Battery. Any time the alarm goes off, your battery will send a push notification to your phone.
If security is your concern, the Nest Cam Outdoor can be up and running in around five minutes. Just download the app, scan the QR code on the device, and affix the magnetic base of the camera wherever you want it.
If you don't want to shell out the cash for a smart cam, you can always repurpose an old phone when you upgrade. Manything, an app for iOS and Android, allows you to download an app and turn that old phone into a security camera in minutes.
For more security, you can always get the August Smart Lock. Sure, it'll take close to 15 minutes to retrofit onto your existing deadbolt -- but that's way less time than pretty much any smart alternative.
To protect against floods, D-Link's Wi-Fi Water Sensor is your best bet. It doesn't need batteries or a hub. Just download the app, connect it to your Wi-Fi, and plug it wherever you want to monitor. Setup complete in 5 minutes or less.
For another 5 minutes, you can install the Chromecast. Combined with Google Home, you'll be able to "cast" YouTube videos (and soon videos from other platforms) onto your TV. Just start with, "Hey Google."
For smaller scale automation, you can always use a smart plug. Most plugs are comparable these days, but the iDevices Switch is one of the easiest installations around. Just scan the code with the iDevices app, and the device will automatically pair with your HomeKit setup.
If you have multiple fixtures to automate, you can use a smart switch. But for all their usefulness, in-wall switches require a lot of effort to install. Enter Switchmate, a device that magnets onto a light switch and lends it a brain.