Third-party devices can add smart features to an existing set, and some of them are quite cheap.
Google’s Chromecast HDMI dongle ($35) is a very affordable way to add some useful capabilities to a TV. It doesn’t add any smart features to an HDTV by itself; but after you install the Chromecast app on an iOS or Android device, you can send content from supported streaming sources—Netflix, Hulu, YouTube, Pandora, Google Play Music & Movies, and a host of others—to an HDTV. You can also send content from the Google Chrome browser on a Mac or Windows system to a Chromecast device, if you’ve installed the free Google Cast plug-in.