Analogue. Not even the merest whiff of digital!
I've never got on with any digital way of keeping track of 'to do' items. For a start, I often need to have them linked to a specific day or week, and I don't use a digital calendar, so that's a bit of a non-starter. And even if I did, the learning curve needed to work out how to have a task assigned to a specific day or week is so much more than writing it on a Post-It note and sticking it in my diary. Or just writing it on the weekly list in my diary.
You'll have seen from an earlier post, that my diary is analogue, with space for 'to do' lists on the left-hand side of a weekly view.
This suits me well for the big-picture things that need doing that week. There's also space for monthly 'to do', which I check in with when I do my weekly planning sessions. If things get super-busy, I either revert to Post-It notes stuck to the appropriate days in the diary, or, if that's getting overwhelming and cluttered, I use a pocket-notebook and have either a page per day or two pages per day with specific 'to do' for that day. My favourite pocket notebook is a Clairefontaine.
Sometimes I keep specific 'to do' in their appropriate notebook - for example, marketing 'to do' (lists of things for a specific campaign or activity) are kept in the marketing book, but this is often because there are long-term things in the list (things that need doing across several weeks) and I would lose them if they weren't all kept together. I generally add information on where those notes are to the relevant diary day (e.g. email campaign next week - see notes on p26 of marketing), just to remind myself there's stuff to check. Likewise, I have a checklist for publishing a book that I don't need to have in front of me every day, but is useful to have in one place and add bits and pieces to my daily/weekly lists when appropriate.
What about all those random notes on the go? The 'to do' that pop up because of a phonecall or an email etc? Or the name of a song on the radio, or a book to look out for? They go in the Clairefontaine. If I'm busy enough that I'm using 1-2 pages per day in that, they just get added to the day's list (and transferred from day to day until they're done). If I'm not busy enough to need the Clairefontaine as a daily list, I use one as a scratchpad and jot stuff down. Each morning I go through it and score things off/review what's on it. I used to use a (gifted) pocket Moleskine for this, but I've used it all up and thrown it in the bin, so there's just the Clairefontaine as scratchpad/daily notes book now.