One of the most climactic boss battles that I have ever run as a DM was at the end of a short dungeon crawl, when the party found that an Adult White Dragon had made its lair in an abandoned forge. The party had gone there at the behest of an NPC to find this forge and make it safe for them to use it again. Long ago (in my home setting), this forge had been created by ancient mages to construct powerful magical artifacts. One of the key components that made this battle as challenging and exciting as it was, was how the dragon used lair actions to restrict the party's movement, forcing them into a funnel for it to use its breath weapon.
This dynamic combat that lair actions contribute to is why I was so excited to finally take a look at Home-Field Advantage. This supplement on the DM's Guild has hundreds of lair actions for more than just high level creatures.
What immediately stands out is that these lair actions do more than deal damage or limit the party's options. They all fit the aesthetic of the creatures that they have been written for. Druids, for instance, can create a cloud of spores and pollen that will either heal or hurt those in its area of effect.
In addition to creating new lair effects for creatures that didn't previously have them, they reimagined lair actions for some existing creatures, such as the Beholder. For instance, the Beholder now has a d6 random effect table for how the magic of the Beholder can alter the target's form for 1 minute. Some of the effects are disturbing and I cannot wait to use them in a game.
All in all, whether you are just looking to use this resource to make combat more exciting, or are looking to inspire your own creative lair actions, this supplement has something for everyone. And with 242 pages, you won't run out of creative and exciting lair actions anytime soon.
You can find the product here: https://www.dmsguild.com/product/384113/HomeField-Advantage--A-Compendium-of-Lair-Actions
And as a DM's Guild Herald, if you use my code: HERALDJOSH5, you can get 5% off on your purchase.