Any suggestions on...what to do for 23 hours in Guam on a Sunday besides getting used to the time change?
You'll probably be wiped out from the travel. Your hotel will likely be in Tumon, the main tourist area on a protected bay, perfect for some relaxed lounging on the beach, renting a paddleboard, or maybe a stroll in flipflops along the beach to one of the old Japanese pillboxes.
A low-key day is going to Hagatna and poking around the Guam Museum, cathedral, Latte Stone Park, and the ruins of the Plaza de Espana, which are all next to each other, before grabbing a meal at one of the many restaurants in Hagatna and maybe seeing what is happening at Chamorro Village and Paseo de Susana. Taxis are crazy expensive, but there are occasional Red Shuttle Buses running from Tumon to Agana Shopping Center, close to the village center.
If you have time and energy for a real day trip, I second the recommendation to rent a car and do a loop down south. You can visit the visitors center and Agat units of the national park, check out the old Spanish cannon battery overlooking Umatac Bay, sidetrip to take the cable car at Talofofo Falls, eat on the beach at Jeff's Pirate Cove, and jump into the ocean at the many many parks and beaches, but particularly Inarajan Pools. There are a stupid number of (not casual) hikes that you can do here. I recommend Mt Lamlam for a nice view, but you'll need a good hat and a lot of water.
If you don't want to figure out what to do yourself and don't mind being the only Japanese or Korean on a bus, Guam is a tourist center and there are many many set excursions (dolphin watching, parasailing, etc etc) that you can just pay to go on.