Your body can definitely get used to doing the same routine and cause you to plateau. If you love the elliptical you could even change up your routine on it, do intervals, etc. Mess around with resistance and incline. Have you ever done one of those stair machines where it's like you're actually walking up a flight of stairs? It looks easier than it is, IMO. You'll work up a sweat, anyways.
Spin is a good cardio workout but I see a lot of people on the bikes at the gym that look like they're going for a nice ride in the park...probably gotta push yourself more than that, maybe in a class, but not sure if that's an option for you bc of the whole "membership" part. Also, you said you don't need any more muscle, but building muscle helps boost your metabolism (muscle burns more than fat!
) Your weight might not go down with lifting weights but you will get leaner and more toned.
How do you feel about working out outside? Running, rollerblading, biking, etc... I personally feel like I get a better workout when I do something outside. Maybe bc I get bored on the cardio machines at the gym and quit early.
I hope this was somewhat helpful! I like to recommend lifting weights bc being strong is fun and most women will not get big and bulky from it because of our hormones.