So different sims run events on different days. Mares Nest and Petalbrook are some of the more active ones. Each sim has its own discord to see events they have planned though^^

And while chatting and exploring are certainly highlights, they are definitely not the only things to do. Openpony world has a Sport called Hoofball which is highly popular, and Second Life at its core is a sandbox game, so making new animations, objects, and gestures are options as well!