I have been looking for a nice standalone GUI reminder tool. Once upon a time one existed: KAlarm. The tool still exists but Akonadi is a required dependency as well as hauling in a half-ton of additional KDE dependencies. The original KDE 3 version lives on with the Trinity Desktop Environment but like KDE Plasma, requires installing a boat load of dependencies.
Sure would be nice if somebody grabbed the code and molded KAlarm into a nice Qt tool with no KDE dependencies.