Guessing it's valve cover gaskets too, not enough information for more diagnosis.
Intake manifold - on the right inner fender the wiring harness bracket can be removed to allow the left hand to guide the socket, knuckle adapter, and extension down onto the difficult side bolt--from right hand coming from straight above. On the Sienna there is a brake type line at the firewall you can follow to find that bolt position, not sure how it is on the Highlander.
You also will likely want to remove the wiper cowel for more room, easy once you resigned to it.
The wiring harness is right down snug to the rear valve cover, and I really lack experience with the fasteners, so I figured the next spark plug change I might change the valve cover gasket being I have no issues. A few instructional video's out there, just watch several and it's probably a little easier on a Highlander.