Read a whole series in the order of publication (or the recommended reading order), without reading other books in between. It’s up to you whether you choose a trilogy or something longer.
I did this with the Anne of Green Gables books – unfortunately I read them in chronological order rather than order of publication, and found a nasty spoiler in the one book that was published out of chronology. I have also done this with sub-series from the Discworld series.
The Harry Potter books might be a not too strenuous series to read like this.
What series would you choose, and why?
What series did you choose and what was the outcome?