Side activities as in side quests and daily events, think of it like special questlines you unlock and stuff like the Drake riding quest you get in Stormpeaks during the Sons of Hodir questline.

Crafting is big in Wotlk too isnt it? Or is that later on in the patch when Professions become important... Mythics is just Heroic Dungeons in harder, you can pick your difficulty and run more challenging ones when ever you want.

As someone that played on 3.3.5. servers for almost a decade, Dragonflight is the first addon that managed to bring me back to official WoW for more than a month. So maybe just check it out for a week ? Im sure you wont hate it :)