Ahhh! I'm Aging!

It is something about our company bathroom's combination of natural and fluorescent lighting… okay, scratch that. It's something about the way I look in any light, or more specifically how a certain part of me looks.

My gray hair is really starting to come in and it's kind of scary.

Genetically it was bound to happen. My father was salt-and-pepper on his mid-20s. I was lucky to have held out this long, as in two months shy of the big 2-8. (Related but unrelated, my boss found an article about gray hair being a build up of hydrogen peroxide. Now what can I do about that?!?)

Whether chemical or genetic, I'm not quite ready to let it all grow out yet. I'm looking at some options, some of which I've tried:

Tried: Professional highlights(beauty school $10-$15, everywhere else $40 plus)
Highlights seem to have the added benefit of changing the chemistry of gray hair by making it lie flatter. But unless I take an afternoon off every 6 to 8 weeks to go to beauty school half an hour away, I cannot afford to maintain them.

Tried: Glaze ($8-$10)
Nicole-grey-hair Glaze is an over-the-counter method to tint and deep condition your hair. It works alright but it washes out, is a little messy to apply, it takes at least three minutes to soak into your hair whenever you use it (ie each time you shampoo). If I don't rinse off the walls of the shower well, it looks like I murdered someone in there when I'm done. Coverage is light to moderate at best.

Tried, though not recently: Boxed haircolor (around $10)
Most people I know in my situation just end up dying their hair. I personally never trust myself to pick out the correct color; I usually instead end up picking the woman on the box that I want most to look like. A more permanent solution than glaze but less maintenance than professional highlights.

Not tried: Henna (around $4)
According to the Internet, I can use henna to cover gray. It seems nice and natural but I found one article that said it would take up to 6 hours(?!). Now that's dedicated vanity!

Do I have other options here folks? Clearly I'd like something inexpensive that won't ruin my hair, bonus if it isn't terrible on the environment. Thanks for any help you can give!