Everyone was in high spirits.
be in high spirits 意思是“情绪很高”或者“兴高采烈”，在这个短语中，spirit要用复数形式spirits表示“心情、心境”，那么“情绪低落”在英文中就是：be in low spirits / be in poor spirits。相关的短语还有：
keep up one's spirits 保持高昂的情绪
lose one's spirits 垂头丧气

We could hardly sit still, for there were so many interesting sights outside the window.
hardly是个准否定词，意思是“几乎不、简直不”；sit still意思是“安静地坐着”，still在这里是副词；for用以解释原因或理由。
sight这里表示景象，例如：a sad sight 悲惨的景象 / a familiar sight 熟悉的情景。sight也有“风景、名胜”的意思，但这时一般用复数形式sights，例如：See the historical sights of Beijing. 游览北京的名胜古迹。
hardly是一个准否定副词，在含有hardly这样的句子中，某些词的用法一如它们在否定句中的用法，例如：
It was so dark that I could hardly see anything. 天太黑，我几乎什么也看不见。
There's hardly any money left. 钱几乎用光了。
Hardly anybody knows him. 几乎没人认识他。

We saw new factories and well-cultivated fields all along the line.
well-cultivated是一个合成词；all在这里是副词，用以加强语气；along是介词，意思是“沿着”；the line在这里指“铁路沿线”。

Our hearts gave a leap, for we knew that in a few minutes we would be with our friends at Anzhuang again.
our hearts gave a leap意思是“我们的心激动得跳跃起来了”。gave的用法我们以前曾经讲过，表示“做出某个动作”；leap在这里是名词，也可以作动词用，比如：Think before you leap. 这是一句谚语，意思是“三思而后行”。

The old villagers kept asking how we were.
这个句子中how we were是一个由疑问词how引导的名词性分句。注意，在这样的分句中主谓的语序是正常的。

They were pleased to see how much stronger some of us had become.
这句中how much stronger some of us had become是一个名词性的分句，作动词see的宾语。
注意，how后面一般要紧跟它所修饰的成份，也就是形容词或副词，比如这里的much stronger就是how所要修饰的成份。
除了how以外，其它一些疑问词比如：when, where和why也可以作这类名词性分句的引导词。例如：
She asked me where I was off to. 她问我去哪里。
I don't know when she'll come. 我不清楚她什么时候来。
Tell me why you gave up that plan. 告诉我你为什么放弃那个计划。

It was getting dark. 天渐渐地黑起来了。
was getting是动词的过去进行时，get在这里可以看作是一个联系动词，意思是“变得”，例如：
He has got very drunk. 他已喝得大醉。
Why do you get angry over that? 你干嘛要为那事生气呢？

Back in the train, we told each other what a wonderful time we had had that day.
“在回去的火车上”英文要说：back in the train，back在这里是副词。
在told each other后面有一个由what引导的分句作told的宾语。为了便于理解我们可以把what引导的分句看作一个感叹句的结构。分句中的had had是动词have的过去完成时。

"Or on the Great Wall," another added.
Or on the Great Wall。这是一种省略的说法，意思要和前面一句联系起来看，也就是说：An outing like this is certainly more exciting than picnicking in the Summer Palace or on the Great Wall.

NOTES TO THE DIALOGUE

Getting Ready for a Picnic
在英文的标题中如果只有动词没有主语，那么这个动词常常是用原型加词尾-ing的形式。如：Going Through Customs。

So you are going to have a picnic in the Summer Palace.
1）so是感叹词，一般用于句首，表示一种语气。
2）注意have a picnic中动词have的用法，这里的have不表示“有”。

What time are you leaving?
这是用现在进行时来表示将来的用法。

You will go boating, I suppose?
I suppose在这里是插入语，意思是“我想”，类似于I think。
go boating是一种习惯的说法，类似的说法还有：go fishing（钓鱼），go skating（滑冰），go swimming（游泳），go shopping（购物）。

The park must be lovely, with all the flowers in bloom.
must在这里表示一种肯定的推测。例如：
There must be something wrong with the computer. 计算机一定是出了什么毛病。
He must be crazy to talk so. 他这样说话准是疯了。
We must have taken the wrong road. 我们一定是走错道了。
with在这里表示“在……情况下”。

Why don't you take a camera with you and take a few pictures?
take a few pictures 意思是“拍几张照片”
take a picture of somebody 给某人拍张照片
例如：Let me take a picture of you.

We'd love to, but none of us has a camera.
We'd是we would的缩合形式。
would love to do sth. 是口语中常用的一种说法，表示“想做……事，愿意做……事情”，类似的说法还有：would like to do sth.
none是代词，作主语的时候谓语动词可以用第三人称单数形式，也可以用复数形式，在这里用的是动词的第三人称单数形式has。例如：
There are none on the table. 桌上什么东西也没有。
None of his brothers is married. 他的几个兄弟谁也没有结婚。
None of them know the truth. 他们当中没有一个人知道事情的真相。

She's got one.
one是代词，代指“相机”；
she's got是she has got的缩合形式。has got 或 have got是口语中的一种习惯说法，意思是“有”，相当于have或者has。例如：
I've got an idea. 我有一个主意。
Have you got anything to say? 你有什么要说的吗？