Thanks for the suggestion. It took me a while to find the fruit I took a pic of yesterday (it was the only one I saw, in fact - the tree has a lot of curled up leaves so I wonder if it is diseased).

Anyway here are the pics of the fruit and the inside after cutting it open:

Avacado (Persea Americana).

Nah, I have avocado trees of my own and the leaves on this one look nothing like it. Plus avocados don't get covered in white blossoms like this one.

Japanese plum (Prunus salicina). This is an orchard fruit so there are a lot of them around.

Example shown is not a very large tree of course. A large tree (of any kind) is one over, say, 60 ft. tall.

Not convinced it's Japanese Plum. The fruits don't look quite right, nor do the bark/leaves (based on a google search on Japanese Plum). Can you show me some example images?

A lot of the immature plums, etc I'm seeing online tend to be narrower at the stem end, and wider at the bottom. This fruit is wider at the top where it is attached to the stem.

Maybe a Pear Tree.

Not a pear, as this has a single seed that'll make it a "stone" fruit. Definitely looks like some kind of plum, and that fruit (drupe) shape reminds me of "damson" plum...

Thanks Shaxhome, I looked at some pics of damson plums online and I think you are most likely right. :)

Does not have the foliage and habit of that species.

As time passes those fruit will become more like their mature selves and easier to id. The fruit being wider at the stem end doesn't stop it being a plum. I can't speak to Japanese plums because I've never seen one.