What is Mandarin's conversational equivalent of English's "How interesting!" or "That's cool" In the course of a conversation in (American) English, I often respond to casual observations and statements using a neutral phrase such as "Oh, interesting" "Oh, I see"  or "That's cool." I've realized that I have never been able to translate this and I often find myself using variations of 真的吗？是吗？to acknowledge what is being said.  

"Hey, I saw Mary at the store yesterday." 诶, 我昨天在超市见了Mary! 

"Oh, cool. How was she?" Umm...Uhh...真的吗? 他怎么样？

I'm basically looking for a phrase to respond to mundane things like "I started taking guitar lessons" or "I saw that movie yesterday" because at least for me, the rhythm of the conversation feels awkward without it. 

Does just saying 真的吗？是吗? work for this? Because I feel like saying this over and over again makes me look a super skeptical person, disbelieving that somebody could possibly buy eggs at the grocery store or see that guy at the bank. Because I don't feel like 是吗 and 真的 translates feel of English phrase "oh really?" does it? Is there a colloquialism that is just as versatile as the English saying "Oh, that's interesting."

But I would want to translate "cool" as a way of expressing that I have some fleeting interest in the conversation. Somebody has translated the slang word "lame" as “没趣" for me... so does it follow that cool is "有趣"? I've seen 趣 in th 对 + Object + 感兴趣 structure, but can you colloquially describe somebody as 有趣 or an activity as 有趣 as well? And could I say it in this way in the situation above?
As for things like 
”I started taking guitar lessons" or "I saw that movie yesterday" you can say 嗯，感觉怎么样？（Oh， how do you like it?) to show your interest in the conversion.
Then they may say,不错（well）、一般（average）、糟（bad）,and now you are taking control of the conversion. You can comment on their response with few words and talk on something else.  

对...object...感兴趣=be interested in ...
If the object is somebody then it's a different matter:you have the tendency to love him/her.
有趣 seems not so native， people often say 有意思 instead. This show that they feel something is interesting. Besides, they say 搞笑 instead of 滑稽 or 有趣 when something is funny. They also say 太扯了 to say something is so ridiculous or amusing.
Some short expressions in English are widely accepted, such as good,what,cool,okay.  

cool can be translated into many Chinese，but it rarely has connection with 有趣. More often it's 帅 or 牛。  

You can use 有趣 to describe an activity as well a person.But here is one point：if you use it on somebody,you have to make it clear that you are saying he or she is humours rather than ridiculous.

Do not repeat one sentence over and over again, that just implies your mind has gone miles away from the topic.