Journey Through the Center of the Underdark has a lot of encounters that have a lot of thoughtful detail, that incorporate both fun and adventure, and that are easily modifiable, with specific guidance in the module for each one.
These are instantly usable for between major plot point encounters in Abyss or really any underground travel. I appreciate the concise booklet, but there's plenty of detail in there to run each one.
This could be a great "Guest DM" encounter set for a party travelling through the Underdark but with a DM who's going skiing next week.
Worth more than the $1 it costs, one of the few products in DMG I've found so far that I've thought this about.