I have an Excel workbook. I also do some bookmarks in the browser or print to PDF some interesting articles of destinations I'm likely to want to visit (or some of those "Top 10 underrated places in..." articles) and save them on a cloud drive.
In my workbook, I have tables that look like this (a past trip as an example). I usually code in only the things I've decided on or booked. The empty space is for exploration and ad hoc things. It's also my contingency for people who I'd like to see but who suddenly need to change our agreed meeting times. The times & booked things I also enter into Google Calendar so they will show up on my phone.
For many destinations I also collect lists of sights I'm interested in. Unless they are specifically booked in, I do those sight more organically as I go. Some days are busier, on other days I soak in the atmosphere and do less. Bold font = I want to do it, grey background = done. As I carry my laptop on my trips, it's easy to maintain the lists on the go (and I could also do it on the phone but that's more tedious).
The approach has worked OK for far. Not perhaps the easiest one, though, but flexible. However, I'm interested to see that would a trip look like on TripIt? And what would an email to TripIt look like if you need to add e.g. a bus connection, a concert, visits to friends, etc?