The Journal of Statistics Education (now called the Journal of Statistics and Data Science Education) maintains an archive of interesting datasets. The data documentation is mostly good, and you can find out more about these data sets in the journal articles themselves. Here are a few datasets that look interesting (I am skipping datasets from this archive that I have already used in class).
2004 cars and trucks data and documentation
What features of a car/truck influence the price or the gas mileage?
Completion times for software projects data and documentation
Does the computer language or database influence the amount of time it takes to complete a programming project?
Calcium measurements in elderly patients data and documentation
Do measurements of calcium and other parameters depend on the age and gender of the patient? Note that there are two versions of the dataset, the first has some of the original transcription errors, I presume to allow students to see if they can spot obvious errors. The link above is to the dataset after correcting the transcription errors.
Electric bills data and documentation
How much does the weather influence home electric bills?
Film reviews data and documentation
Do the length, number of cast members, or release year have an impact on the film rating?
Fish catch data and documentation
How much do size measurements vary among different fish species?
Frutifly lifespan data and documentation
Does the lifespan of a male fruitfly depend on his sexual activity?
Used cars data and documentation
What features of a used car influence its sales price?