Having a broad, clear, unobstructed path to the waterside patio would be nice ... where each step is at least 5' or 6' wide. Since there are rocks laying everywhere, it's a potential DIY building material, if anyone would have that inclination. If you want to level anything, I's suggest limiting it do the lower patio or the patio area at the immediate back of house. Everything in between must be taken up with slope, or it could be terraced, but that would cost a fortune for getting no more than something pretty. You could make it pretty without that.

Areas not taken up by patio or dedicated path can be groundcovered so as to discourage their use as walking places, and to smooth out all the visual roughness that the yard has now. They can be part of an improved view. If you make a "tree adventure, have a dedicated path to it as well. It's a yard where people are better off staying on civilized paths.

You could have some tall shrubs/narrow trees strategically placed along the side lot lines in order to give a sense of privacy.

It looks like a great spot by the lake, but also it seems the seating area and dock are the only serious efforts at utilizing the yard.

Clean up the joints in the patio, maybe dig them out and then top them off with some new stone dust. They're kind of big so I don't think polymeric sand would be any good, just use straight stone dust.

The loose stones scattered about seem messy. Either stack them for a real stone wall effect or move them to the edges of the property.

There is no way to avoid the tree roots other than cutting the trees and removing the roots or covering them with a thin layer of soil. Thicker soil may smother the roots. Then you'll need something to keep the soil in place. Maybe instead, put down a thick layer of bark mulch. That would help with the tripping problems, but will be temporary.

Get or build a canoe rack so it's not always thrown on the shore.

Check with your association on what you can and can't do as far as a 'beach'

Not much you can do with the trampoline.

Plantings will be iffy. Your soil looks thin, mountain laurel and rhododendron might work, but I'm going to guess there are a lot of deer. Look and see whats doing well in your neighbor's yards.

As far as a tree adventure, utilize the underside of the deck. It won't be as visible and it's otherwise wasted space. Look up a few images for under deck obstacle courses and depending on the ages you can go from there.

You need some more defined paths, right now they're a mix and look like tripping hazards. You may need steps put in so that everything doesn't just wash downhill onto the patio.

Hope that was somewhat helpful, it kind of sounds like a bunch of complaints!

Good luck.