How do you structure your school day when you have more subjects to teach than you have class periods?

I'm a first grade teacher, and I have one 30-minute period per day allotted to science, history, and writing. I currently teach writing on Monday, Wednesday, and Friday. Tuesday and Thursday, I do history first semester and science second semester. The disadvantage to this is that it doesn't provide for much continuity in the subject and that writing a single story can take up to three weeks to complete (especially when you consider that we have to do prewriting, drafting, revising, editing, and final copy of each writing piece).

I've wondered if there is a better way to structure the school day. I'd be particularly interested in hearing about research regarding the most effective way of teaching when you have more subjects than class periods. Methods I've considered:

1) Keep things as is: Writing 3 days/week and science or history 2 days/week. Advantage is that students get writing all year long.

2) Teach writing first quarter, history second quarter, writing third quarter, and science fourth quarter. The advantage to this is that we could finish a story from prewriting to final copy in just over a week, so the students wouldn't lose interest or feel like I was dragging out one assignment forever.

3) Incorporate writing into the science or history curriculum as thematic units. For example, when learning about plants in science class, have the children write directions on how to plant a garden. When learning about Pilgrims, have the children pretend to be a Pilgrim and write a letter to a Native American friend. (The disadvantage to this is that our school uses a structured writing curriculum and it wouldn't adapt very neatly to thematic units. I'd more or less have to drop the textbook and just teach the methods on my own, which I could certainly do, but I'm not sure the administration would be okay with it.)

If you're an elementary school teacher with a similar situation, I'd love to hear what you do and why. Thanks!