I think a bonus for Lab Adventures are smart for two main reasons:
1. Make sure people are aware that there is a Lab Adventure in the area. It's in a different and if you don't check that app to see if there is any in the area, you might miss it, a bonus helps prevent that.
2. Feedback, although in a recent update to Lab Adventure, you can now leave feedback in the app, you don't get notified as a CO, so I stumbled upon some feedback on my Lab Adventure when I was checking how many had solved the stages.
I decided to make my Lab Adventure bonus cache as a Letterbox, where you get 1 direction per stage in the Lab Adventure, the last stage gives the code for the lock. Making it a Letterbox makes it stand out a bit from all the mysteries, that tourists may ignore due to not having them solved from home. At least that was my reasoning for figuring out how to do my own, and people have responded kindly to it so far. :)