How can I greet a person in a manner similar to "Nice to find you in here!" (— meaning meeting a friend by chance somewhere)?
There might be a lot more ways to say that, but I’ll try to offer more general examples. The following lines would be said with a wide smile.
あら、こんにちは（or どうも）！ (Female speakers would say to their friends.)
よ、どうも（or こんにちは）！ (Male speakers would say to their friends.)
In my opinion, Japanese people don’t say a counterpart to “nice” because they express it with their smile. They don’t bother to say it. 「あら」and「よ」are said to draw an attention. (They might express a pleasant surprise.)
I’m not sure how often speakers like こんにちは or どうも, but as far as my family is concerned, my husband, 40s, don’t like long words. I like longer words, for some reason. (It may depend on speakers’ sex, but I don’t know for sure.)
Well, just for your information, what about meeting superiors?
I would say
My husband will probably say
The above lines would be said with a wide smile and a bow. I personally think どうも adds a little polite tone to the line. I feel this どうも is different from the どうも in 「よ、どうも」which seems simply to mean “hello”in a friendly way.