conlangs, reddit (~)
There's lexicon-building challenge happening this month on reddit, and I think the challenge looks really well made:
https://www.reddit.com/r/conlangs/comments/v4rusx/its_junexember_again/
I don't think I'll have time to do it this month (I'm super busy for the next 1.5 weeks), but... it looks like a great challenge: multiple prompts, and rules to encourage you to do more interesting things (words with multiple meanings, homophones, deriving words from other words, etc)