Having very recently put myself in the FIREing line, I too have found myself wondering what I will do to fill the days in a couple of months time when work no longer steals them away.
First thing on my list is to unwind. After a 30 year career, and all the sacrifices that entails, it will be great to just kick back my heels and spend quality time doing absolutely NOTHING. I don't know how long it will take for that novelty to wear off, but on the other side I have a bulging "honey do" list to tuck into.
That'll take care of the extended vacation phase, but obviously that too will pass. Next up will be the list of longer term activities that I've always wanted to try, including the good old bucket list. The content of the list is not so important, as it will depend very much on the individual, rather it is the fact that I have been consciously growing it as I entered into the FIREing zone.
Finally, I have a contingency plan for returning to the voluntary or paid workforce if ER is not for me or my FI finances run aground, thus requiring me not to burn any bridges on the way out that I may need to traverse to get back in. I really don't think it will come to this, but my last 20 years in management forces me to consider the consequences of the risk, regardless of how unlikely it may eventuate.
I truly don't know how any of this will pan out, but IMO that's part of the excitement and challenge of FIREing. Although it may sound as though I have planned my ER in detail, the reality is that my choices in what I do will be far greater than I what I can achieve now and my selection of them will be unscripted (for the first time in my otherwise well-organised life!).