I just signed the paperwork for my Odyssey EX. Thanks to reading this site, I got nothing installed by the dealer.
As mentioned, I'm ordering some of the popular items from one of the websites listed in the Links section of the website, much cheaper, and installation for many of the items is very easy, taking only minutes. Meanwhile, the dealer was charging up the wazoo for installation. $30 to install wheel locks!!! Over $50 to install crossbars!!! That's on top of the price of the items, which he was giving me at cost.
In any event, the things I think will be worth it: cargo tray, splash guards, wheel locks (handa-accessories is offering wheel locks for $10 when you get the cargo tray and splash guards).
In the future, I might get the wheel well liner, crossbars (decided I won't need them for a while), and I'd like to get the cassette player. Even the cassette player, after reading the instructions, seems like a piece of cake to install. The dealer wanted over $230 installed, handa-accessories sells it for $132. His excuse? "Well, you get the 3 year 36K mile warranty on it." Of course, by doing it myself I can afford to buy a brand new one should the first one fail.
Non-Honda items: new speakers.