Here are some ideas:

• Do other stranger missions

• Complete compendium and all challenges (Get Legend of the East outfit)

• Find all gang hideouts

• Commit genocide

• Explore New Austin & Tall Trees

• Pay respects (find gang members’ graves)

Once you reach 100% completion however, there isn’t much left you can do. I recommend restarting the campaign after 100% completion or finding another game to play (possibly Red Dead 1)