How I’ve combatted my own personal boredom:
1) playing Mason (solitaire d6 game) while paying attention. And then, only after I’ve planned my next action.
2) if permitted by table etiquette, helping others with their PC decisions.
3) campaign/PC design for future games
4) contemplation of current PC advancement options
5) designing mechanically complex PCs that require more effort to run properly, leaving less time to become bored
6) like 5 above, but with complex roleplaying aspects that require constant attention.
7) trying to divine the overall campaign plot.*
Above all, I try not to be disruptive, and as noted in 1, I do my best to be prepared for my own turn, regardless.
* I did that once, shockingly early. But events conspired to have the campaign fizzle out.