How can I greet a person in a manner similar to "Nice to find you in here!" (— meaning meeting a friend by chance somewhere)?
The most common one is
'It's coincidental (as if it were planned).'
but it does not particularly mean you feel nice about it (nor does it mean it is bad). If you want to express that, you can just add the direct translation:
'I am happy to meet with you again.
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.
（ここで）出会って よかった／うれしかった （です）！
I would go with お久しぶり！ because it works even if you saw them 2 days ago.