Pen in bag

Never take a pen with you in your bag just like that, but use something to prevent it from leaking.
Ofcourse you can wrap it in kitchen paper and put it in a plastic bag.

But you can find a container of bubbly vitamin tablets and use that for your pen.

Use coloured tape, stickers or something else to make the container a joy for your eyes, you can even use a cord to connect it to your bag.