I just changed the six spark plugs of my 2000 Sienna a couple of weeks ago at around 85k miles. It's not that easy to change the back three plugs, but I succeeded to replace them without removing any parts.
There're several how-tos found on web, it's very helpful. They gave me confidence on how to do the job.
Tools I used: a 3/8 rachet, a 6-inch 3/8 socket extension bar, and of course, the 5/8 spark plug socket. I used a 3/8 stanley rachet bought from Walmart, still a little big. Later I found a very short husky rachet from home depot, it would be great if I had used this husky one.
The front three are very easy. Since you can't see the back three plugs, it's better to locate their position first. I just put my hand to the back of the engine and found the three boots before I even started, so that I know it's doable for me (My hands are not big, an advantage here), then I used my daughter's small mirror to check what are there and made sure I was right.
When you work on them, you have to learn your body on the engine part, so be sure the engine are cool down at that time.
I changed the right one first. First pull off the boot, then slowly slide in the 6inch extension with spark plug socket, then attach the rachet. Took the old plug out, put the new plug in. The middle one is the same thing, working from left side. The left one took more time, 'cause there's lots of wires there. Had to unclip the spark plug wires first.
It took me around two hours, lots of time spending on blindingly sliding in the socket, finding the holds, and attaching rachet. It needs patience and perseverence.
I used NGK iridium plugs and it's great. My wife told me the next day that she could feel the engine much more powerful than before.