Do not use Hidden Power Dragon. It provides terrible, redundant coverage and the slot can be used much better with something else. It really depends on the needs of the rest of the team, so choose accordingly.
Brave Bird. Due to Tapu Koko's high Attack and Brave Bird's high Base Power, it still deals a lot of damage even without a lot of investment. It also provides good coverage, hitting Grass types which resist Thunderbolt but more importantly Grass/Poison types which resist both of Tapu Koko's STABs, such as Amoonguss and Mega-Venusaur. If you do choose to run this, put the four Special Defense EVs into Attack. Use this if your team is more offensive in nature, as it allows Tapu Koko to deal more damage and not get forced out as often and lose momentum.
U-turn. It might seem redundant to run both U-turn and Volt Switch, but there is good reasoning behind it. Volt Switch should be used most of the time, as it still does decent damage. However, if there is a Pokemon that can block Volt Switch (either a Ground type or a Volt Absorb user) then use U-turn instead to avoid the loss of momentum. Plus, U-turn allows you to escape from Dugtrio should it trap you. This can be used if you have frail but powerful Pokemon which appreciate the free switch-in it can provide.
Taunt. Pretty self-explanatory. Allows Tapu Koko to pressure defensive Pokemon by cutting off their access to utility moves, which is useful as Tapu Koko can sometimes fall short with damage. If your team lacks powerful Pokemon that can break through defensive cores then this is probably the best alternative.
Also, I highly suggest replacing Dazzling Gleam with Nature's Madness. Dazzling Gleam is pitifully weak as it is not boosted by Electric Terrain and has low Base Power. More often than not, the combination of Nature's Madness and Thunderbolt will hit switch-ins harder than two Dazzling Gleams or a Dazzling Gleam and Thunderbolt.
