You shouldn't look for interesting Lemmy instances; you can pretty much use any instance and subscribe to interesting _communities_ from there.

If English is not your native tongue, I'd suggest you to find an instance in your language, so you can easily see all kinds of content if you filter by "Local".

Some random active communities to follow:

- https://lemmy.world/c/selfhosted

- https://mander.xyz/c/science_memes

- https://feddit.org/c/europe

- https://lemmy.ca/c/pcgaming

Depends how you define "strong activity". Its only 50k MAU so i'd say its punching far above its weight in terms on good content/users. The tech(selfhosting, linux, privacy) communities are pretty good, you make a post you will get answers and discussion.