Do as many pull ups and chin ups as you can. Until you know you won't be able to complete the next one. Rest for between 1 and 2 minutes and do it again. Do 3 or 4 sets. Pull ups target the brachioradialis more than the biceps. Chin ups target the biceps. Both need to be strengthened for overall arm health and strength. Both exercises target the lats. And rear deltoids. You can also do neutral grip pull ups/chin ups if your pull up bar has the handles for it. I find that chin ups hurt my elbows because my arms are strained in the fully straight position. I never do chin ups now. Only pull ups. Doing chin ups on rings is kinder to the elbows as you can turn your hands a bit.
If you can do more than 10 pulls or chins and don't want to add extra weight by wearing a knapsack or tying weight round your waist you could do L-sit pull ups. Or pull ups with your upper legs parallel to the ground but lower legs vertical. Easier than L-pulls.
You could also try tuck front lever pull ups. You can see videos on Youtub of how they are done.