Rachel and Kurt are living in NY (I think Blaine also lives in NY, but the three don't know each other.) and take a vacation at a lake house somewhere. There they meet Blaine, who has rented the house next door (I think they meet when Blaine is out running and Kurt is just getting out of the house?). Rachel starts inviting Blaine to dinner so he won't have to eat alone. Kurt and Blaine eventually get together but agree at the outset (?) that it will be a vacation fling. When the time comes for Kurt and Rachel to head back to New York, Blaine leaves as well. Unbeknownst to the other Kurt and Blaine each slip their contact info into the other's book or luggage. (I think one of them had a journal and the other maybe had a novel which were used as the hiding places). I'm fairly certain one note was written on a gum wrapper.
The artwork was, I believe, a pencil sketch with Kurt sitting on the dock and Blaine was laying down with his head in Kurt's lap. there may have been bonus art as well, but I don't remember the details of it.
Found: Three Weeks of Summer series by alianne. The one I was looking for is the first of the series.