Harry has several gifts for Hermione, which he will deliver to her room after school while she isn't in it so she can come in to be surprised by the gifts waiting there.
On top of the wrapped gifts is a card, 'Hermione, I got you a few things, one for you, one for us, and one for your smaller form.'
There is a kiddie swimming pool for when she shifts into an otters as he knows how she enjoyed swimming in the sink and the tub, and he thinks if she's forced into the otter shape again, she will really like the kiddie pool.
No, he could not get a simple one. He had to go with one that was a bit more complicated. He thought she'd enjoy it more, alright?
The for us portion of his note referred to the cake that he left in the middle of the pool. It's her favorite kind of cake or sweet, whatever her favorite thing is to eat, cake or pie. It is there for her with candles on it ready to be lit.
And finally, there is something for her. He considered going for something that was book-related. A fancy bookmark, a reading light, something of that nature. However, she has all the books she could need, all the things for the books as well, and he wanted to get her something that would help her relax while studying and doing the in depth research that she does.
To find a way to help him.
So he got her a relaxation fountain for her desk. It has LED lighting, soothing water sounds to help create a sense of 'relaxation'. Harry doesn't really understand half of what the saleswoman was telling him about it, but the trees remind him of the forest that she took him to, the one that she went camping at with her parents.
She doesn't know she took him there, and he doesn't say it because she didn't know and he knows how she feels strange about the time she didn't live with him but another Hermione (and it is still strange) did.
But the trees like that remind him of that place. It's beautiful, and it reminds him of her too.