Although I use the computer for most things, do not do so for my diary.
Like Lemongrove, just have the one pound ones, which is always kept close to me. Next to me indoors, and in my handbag when I go out.
Not only is this used for ALL arrangements (one of my g.children laughed when she noticed I had even put down which day I intended to do my washing!!), but it also lists useful tel. nos, full details of medication I take - so useful when I get asked, I just hand them this diary open at the appropriate page. Also, have all the numbers of important items (Blue Badge, NI No., Passport No.,Driving Lic. No. etc. etc.).
Have just filled in all of this in my new 2020 diary, along with birthdays /anniversaries of all family members and the year they were born/married.
I do take great care of this diary, totally lost without it, but can lead a reasonably ordered life with it.