I'm not sure that it's ever realistic to call River "sane" at any point except in the flashbacks. In addition to having her mind screwed with by The Alliance, she has horrific memories embedded in her brain that she doesn't even know she has. Mostly, she goes through periods of lucidity followed by periods of disturbed behavior, which is not unusual for someone with certain kinds of mental illness. About the best I think we can say for her overall mental health is that she got much better after the events of Serenity. (Caveat: I have not read the latest Firefly graphic novel series, which AFAIK is the first to take place post-movie. Presumably that will have more information about River's current condition.)
However, if you're just concerned about those periods of when she's acting normal behavior, there are plenty of them. Usually, when she's relaxed, she seems to be a normal, pleasant teenager. For example, she is often seen playing games with Kaylee in a very much big sister/little sister way. There's a scene earlier in "Objects in Space" where they are playing jacks, I believe (just before she has an episode with the gun). There's also a scene early in "War Stories" where she's being chased around the ship by Kaylee for stealing an apple.
Another example of behavior that's sane, though incredibly creepy, comes at the end of "War Stories", the famous scene where she kills 3 of Nishka's men while firing blind. She's acting very cold and calculating, but that's not much different from what Jayne or Malcom might have done in her place.
Finally, any sane person would likely react by screaming and running away at their first glimpse of Shepard Book's real hair.
