I agree, CO1 goes via IAH to GUM with a stop over in HNL (about an hour and a half), CO2 goes GUM to HNL to IAH. I'm not sure I'd go through Japan at all on this. Unless you want the extra miles of course.

But if you do and have your layover in Guam that long you have to get your bags as it's a different customs and immigration system then the US as it is a commonwealth and they operate their own system there, and you will be coming in from a seperate foreign country (the FSM).

I'm going to take off next week from here (the island hopper) and transit GUM while taking CO2 back over. If you want a cheap decent clean hotel look up Harmon Loop. It's near the airport, decently clean, cheap at 50 bucks a night, across from a McD's, has a little convient store right in it's parking area, and is near a decent restaurant called Shirley's (nothing fancy but decent food at a reasonable price). Harmoon Loop is where a lot of the local Micronesian's stay when they transit GUM.

As far as what to do you can go visit the worlds largest KMart which is in Guam, or go drive around the island. There are some nice places to stop and see the beaches and such. There's the main drag of hotels that has some nice shopping and views of the ocean but are geared toward Japanese tourists but you can walk around that area and such.

If you head over to Harmoon Loop they have a shuttle that operates certain hours but I normally grab a cab anyways. If you want a rental car and are there Thrifty will pick you up and take you to get the car.