这里的 a 要不要？a reading one of my favourite books.

我认为不要！我认为是
[Meeting] a friend that I had not seen for a while
or [receiving] a surprise gift
or [reading] one of my favourite books.
这三个动词的现在分词做主语，并列表示那些生活中简单而令我们感到愉快的事情。其中的reading和上文的meeting\receiving一样，不用加a。

凭感觉看了一遍，觉得不用a好些，不过你的句子里太多that了，已经不好了，再加不加a都一样。
我给你写了个，参考参考。
I believe what keeps us from being truly happy is quite simple really, e.g., meeting a friend long time no see, receiving a surprise gift and reading a favourite book.

//注意，这里用的是and,不是or.