Buy a Block Plane
Block planes have become one of the most oft-used tools in a woodworker’s workshop. Some traditional woodworkers even keep them in their aprons! These little planes can be used to trim your joints, put chamfers on board edges, trim end grain, etc. I would recommend finding a low angle block plane, because the low angle lets you cut difficult grain more easily.
I personally prefer a low angle rabbet block plane because it allows me to trim right up against a tenon cheek or other joints, but this isn’t required. My handplane buying guide goes into more detail about the features and brands that you should look for when purchasing a good quality block plane.