I'll drop by later
I'll drop in later
He came by last night
He dropped in last night
He just popped in, were having a beer now
For #2 many things could be used and it depends on the situation. How "wanted" or "unwanted" the person is. Some words have very negative connotations, like in eastern Canada one might say she is a clingon/cling-on, and this is not nice.
Sorry i can not come up with any idioms. Surely there are lots.