After getting a bike and riding again a little over a year ago I've bought: 1) Padded liner shorts, as they make rides more comfortable (I mainly wear standard workout clothes over them). 2) A bell as hikers/runners with headphones weren't hearing me give notice. I think it's been more effective in alerting animals, though. 3) Front and rear lights for early morning and evening rides. 4) Small bag with a spare inner-tube, multi-tool, tire levers, and patch kit. 5) Water bottle and cage. 6) Maintenance and cleaning supplies. That's been it, except for replacing worn parts.
I've been using a couple GPS/tracking apps, though. I may get a bike computer, especially as I want to explore longer rides in unfamiliar places. And I plan to convert to tubeless at some point.