If you're just interested in mileage expiration keep it simple. I use a spreadsheet with just the program name and expiration date (usually three years after last activity). I then sort by date and can easily see which programs need activity.

I use mileage expiration date instead of activity date because different programs have different expiration rules. For example, DL miles expire on Dec. 31 of the year in which miles are to expire as opposed to three years after last activity.

Also, I don't bother including my primary carrier that I fly every year since those miles effectively won't expire.