When I want to freeze portions of almost anything liquid (including stocks and sauces), I use a covered ice cube tray like OXO Good Grips. Just spoon it into the tray and throw it in the freezer. Easy! Keeps very well, and makes it very easy to portion out later.
The OXO product is my favourite, but it's not too hard to find generic substitutes at any superstore. Try to find one with a soft bottom so you don't struggle for 5 minutes trying to get the cubes out and then scraping the sides for all the bits you left behind.
It depends on what type of lettuce it is -- part of the issue is that if the lettuce is touching plastic, it will rot quicker, so I wrap it in paper towels, then bag it (but not sealed), and keep it in my crisper.
For whole heads of lettuce (iceburg, butter, red leaf, etc), I just wrap the whole thing in dry paper towels, then shove it back into the bag from the grocery store or farmer's market. I then pull off leaves as I need it, and re-wrap it. It stores for well over a week this way.
For mescalin mixes, arugula, or other individual leaves, I'll wash them, dry them, then unroll enough paper towels to spread the leaves on, then roll up the whole thing, and bag the roll (again, not sealed), and keep it in my crisper. I can probably get a week out of it this way.
(all times assume you're not buying from a store where it's been sitting on the shelf too long before you buy it; I get my lettuce when I can from the local farmer's market)
So, to answer the specific questions:
- keep the bag open; you don't want moisture to condense inside the bag, as it'll make the lettuce rot faster.
- I get better storage time with heads of letuce keeping them whole. If you're going to be eating it all within 2-3 days, it probably doesn't matter, and for loose lettuce, I find it more convenient to wash it as I re-pack it anyway.
- You never want to store lettuce wet ... you might be able to store it completely submerged, but damp will lead to it rotting faster.
Best Answer
Yes, cakes will go stale faster in the refrigerator, as will most baked goods with a high starch content.
Staling (as discussed over at Serious Eats) has to do with starch recrystallizing. As Harold McGee explains (pp. 541-542 of On Food and Cooking):
Basically, when you mix starch and liquid together in a dough or batter and then heat it, its crystalline structure breaks up and it absorbs water, giving bread and cakes their "moisture." After the starch cools, it gradually goes back to its original form, thereby expelling moisture and hardening, producing the quality commonly called "staleness."
All of this is just as true for the starches in cakes as they are for breads. McGee goes on to confirm that staling recrystallization proceeds most rapidly at temperatures just above freezing. He also notes that certain emulsifiers will retard staling (such as egg yolks and true buttermilk). I would add that in cakes, the higher sugar content and the fact that sugar is hydrophilic can help cakes retain moisture longer. Hence, some significantly enriched breads and richer baked goods like cakes tend to stale more slowly than lean breads.
Nevertheless, the chemistry of staling in cakes is still basically the same, and it will happen faster at cooler temperatures as long as they are still above freezing.
Thus, if your primary goal is to prevent staling, storing cakes and quick breads at room temperature is best for a few days. For longer storage, wrap tightly and store in the freezer rather than the fridge. (Staling happens very slowly below freezing.) As moisture loss outside the cake will accelerate moisture migration out of the starches (and thus staling), covering unfrosted cakes and cut surfaces is essential.
Of course, staling is not the only concern in shelf-life of cakes. Some frostings may not do well sitting for days at room temperature or particularly in hot weather. Some may even become unsafe. Cakes or quick breads with a high moisture content due to added fruits, vegetables, etc. may also begin to grow bacteria or mold when left at room temperature for several days. In those cases, you have to balance the benefits of refrigeration in preventing spoilage with the minor detriments of staling. (Obviously if it's a food safety issue to leave a cake out at room temperature, it's best to refrigerate or freeze it for storage.)
What to do if your cake has begun to stale (either due to refrigeration or merely sitting at room temperature for several days)? The starch crystallization process is mostly reversible, so heating the cake above around 140F will begin to restore the cake to its previous state (as it will with bread). Obviously not all types of cakes (especially frosted ones) may allow such reheating, but it can help with quick breads and simple unfrosted cakes. However, reheated cake should be consumed immediately, as the interior of a stale cake has already been somewhat depleted of moisture, and a cooled stale cake will become progressively worse (hard and dry).