Awesome job in venturing outside of your usual style and comfort zone, you did great! There's a very light and whimsical vibe for the first half of the album that transitions well into the slight shift in tone in "Wonder by the Aquarium" and finishes strongly with the emotion and energy in "Last Summer Night."
Looking through some of the feedback you've already gotten - I would add that the drums feel a bit heavy on the low-end on the reverb side in "Last Summer Night." A simple trick for EQing for beginners is to just cut out all of your low-end reverb (just the reverb and not the whole instrument) at ~200hz and below. It's not perfect, because there is no such thing as a one size fits all technique for mixing, but it might help you out for the time being. Regardless, this is ultimately a pretty minor issue that doesn't detract too much from your OST.
I don't think I have much to offer for tips on making ambience because I'm awful at it lmao (but I tried my hand at some sort-of horror ambience of my own on one track in this jam and I think it turned out kinda cool?) Either way, I'd say "Wonder by the Aquarium" is definitely pushing it on being classified as ambient, but it still gets the intended feeling across very well and I wouldn't let that "is it or isn't it" classification hold any weight or bearing on the quality of the track - it's good! If you wanted to actually make it ambience, you'd probably have to reduce the emphasis on your melody quite a bit, which I think would change the track for the worse.
Overall, this is a great submission! Again, nice job in stepping out of your comfort zone and keep up the good work!