My kids rooms are connected by a Jack and Jill bathroom. When my daughter decided she wanted her own bathroom a few months ago, we had her "take over" the guest bathroom. You can check out her new bathroom here. This is what I did to the Jack and Jill bathroom when we first moved in:
I picked a gender-neutral theme of fish. I took the fish directly from the shower curtain and bath mat and hand painted them onto the wall. I then had two fish hanging from fish wire from the ceiling.
I changed out the builder light fixture above the sink with a light fixture my son picked out. The light fixture was purchased at Lowes.