Thank you, everyone, for all the great ideas. We have 2 showers, but the gpm isn't listed on either. They're both old enough that I'm sure they're more than 2.5 gpm.
It sounds like I probably won't like anything less than 2.5, but I like the idea of an adjustable, so I may go with the one Mr. Frugalwoods suggested. The lower setting is 1.85, but DH takes such quick showers that there probably isn't that much difference in water usage over a 1.5. There's also a 20% off promotion on Amazon, so it will be less than $20. Holyoak's suggestion also sounds good, but I think it's a little beyond my abilities right now.
The boys' bathroom is a tub/shower combo and has an old WaterPik shower massage. I wanted to keep a handheld in here because it's helpful when washing large objects in the tub. I had been eyeing the one Gumbo1978 suggested, so I'll probably go ahead with that one.
I plan on changing my sinks as well, but I'm starting with the showers because we use a lot more water with the showers than with the bathroom sinks.
After I change the showerheads, I'll have to work on adjusting the water temp. I assume if we use less water, I won't have to keep the temp as high. I'll definitely wrap the water heater, too. I have no idea why we haven't done that yet.