I have loads of sewing tools and notions some intended for sewing others not, but that work well in the sewing room. One of my essential go to tools, and one that drives me mad if I cannot find is a corner turner or point turner.
My favorite is actually a notion that is specifically made for this, it is white and smooth with a rounded edge, I actually have 3 of these so the chance of losing them all is low. I also have blue one with a pointy edge, a ruler and a button guide, I use this too, less often.
I also use other tools for this task, firstly a chop stick, this is good for reaching into longer spaces when my hand will not fit. Just be sure if it is wood that there are no splinters that could snag fabric, a nail file will help eliminate those rough spots.
Pencil, in a pinch I use a pencil, I stick with a white marking pencil as it is almost never "sharp" and will not leave big marks like a lead pencil.
Below is a nifty stylus that I got as a paper crafting tool, it has a very fine ball point. This is intended to work with embossing plates from Fiskars, I actually have no clue where my plates are but this is a good tool for tight corners.
Perfect corners are essential and easy with my many corner turning tools!