This accessible, free and fully online introductory textbook in linguistics has videos and exercises that are not only informative but also fun.
Glottolog is an amazing resource for showing the diversity of languages around the world and how they are related. Pick an area and see how many languages are spoken there. How many of the languages you find have you heard of? Or: pick a language you have heard of (like English or Welsh) and see which languages it's related to. How many of their relatives are ones you haven't heard of?
The Language & Life Project at NC State have made a bunch of documentaries available for free on their YouTube channel:
NativLang includes a range of videos on different language-related aspects. For example, you can try pronouncing some of the rarest sounds in a number of languages, find out what old languages sounded like or find out a bit about the history of writing.
You can find out about different languages via Langfocus on YouTube. These videos include answers to questions such as what are the differences between Mandarin and Cantonese or how similar are Persian and Arabic, but also videos on individual languages, including constructed languages.
Interested in sounds? You can look up the project website for the Seeing Speech project (where you can find an interactive IPA chart and even see the vocal tract movements with MRI, ultrasound or an animation.
The International Phonetic Alphabet can be used to write the sounds of any language in the world. You can listen to them here.
There’s also the Dynamic Dialects project - an interactive articulatory accents of English database, which you can find Dynamic Dialects. Dynamic Dialects contains a corpus of speech samples from world-wide accents of English. articulatory video-based corpus of speech samples from world-wide accents of English. Videos in this corpus contain synchronised audio, ultrasound-tongue-imaging video and video of the moving lips.
Visiting this resource from the British Library, you can explore the diversity of Accents and Dialects in the UK. Among many other thing, you can learn how speakers use English differently according to age, gender, ethnicity and social or educational background or find out more about Geordie connected speech. British Library has also an extensive collections of unique sound recordings, which come from all over the world and cover the entire range of recorded sound: music, drama and literature, oral history, wildlife and environmental sounds.
The Lingthusiasm podcasts include lots of interesting discussions by and for language enthusiasts. You can hear about the similarities between the way we use emojis and gestures and about making sense of silence and invisible units of meaning. If you’ve ever wondered whether we can smell words, you’ll find something for yourself here, too.
If you want to try yourself at linguistics puzzles, the Linguistics Olympiad has put lots of problems online. Here for instance are 2019, 2018, 2017. Problems for fun that often lead into to undergraduate linguistics.
On the daily blog All Things Linguistic, you can find language and linguistics-related podcasts and lots of other resources. For example, you can find out about rules of splitting words with “infixes” in the English language.
"…you’ve never been taught these rules because some of those infixes are words
you probably shouldn’t use in front of your high school English teachers…”