You might want some fractional plates and as someone else mentioned, adjustable dumbbells or the olympic dumbbell handles.

If you get the handles you'll want to pick up a couple more 2.5lb, 10lb and 25lb plates so you have four each. The fractional plates will work with these too as you can combine the .75s with .5s and 1s with .25s in order to increase dumbbell weight by 2.5lb increments (1.25 per side for each dumbbell).

A curl bar wouldn't be bad if you don't like curling with the flat bar. It's nice for skull crushers and some other exercises as well.

An ab wheel or some ab straps to hang on your rack to do core stuff.

Assuming you go to the gym...

More than $20 but totally worth it.

https://www.amazon.com/Ader-Fitness-Olympic-Fractional-Plates/dp/B008RP3KY8

ABC - "Always Be cProgressing"