perhaps a quote from David Allen (Getting Things Done
, p. 194) would illustrate why this particular concern of yours is not such a big deal:
Just as having all your next-action options available allows you to take advantage of various time slots, knowing about everything you're going to need to process and do at some point will allow you to match productive activity with your vitality level.
I recommend that you always keep an inventory of things that need to be done that require very little mental or creative horsepower. When you're in one of those low-energy states, do them. Casual reading (magazines, articles, and catalogs), telephone/address data that need to be inputted onto your computer, file purging, backing up your laptop, even just watering your plants and filling your stapler—these are some of the myriad things that you've got to deal with sometime anyway.
The stuff you put on the low energy list is almost never going to be the "optimum" task you could do, except if the alternative is doing nothing at all. If there's some eventual urgency to getting it done, put a due date on it, which will bring it to your attention if you haven't just done it by then. Or catch it in your review process. You do use the review feature, right?
You shouldn't confuse low energy with short duration, as they are completely different concepts. A low energy task is not necessary a brief task, and a brief task may not be a low energy task, either. Though the iPad app doesn't allow you to set or view duration information about a task directly, it does allow filtering, sorting, and grouping of duration via perspective, and if you bother to put in duration estimates for all of your tasks, you'll probably want to set up some perspectives that show all available actions variously filtered, sorted and grouped by duration. Make one for available tasks, group by context, sort by due, estimated time of 5 minutes and you've got a list of things that you can quickly examine and just do something. Do a handful of them and you might find yourself back in a higher energy mindset, ready to tackle bigger things.
An important idea behind GTD and contexts is that it allows you be more productive by spending less time deciding what to do next. You don't want to lose that efficiency by spending all of your time fooling around with complicated searches, applying every conceivable tag, multiple contexts, etc. Use reviews, flags, and impending due dates to keep the important stuff moving along, and fill the remaining time with everything else.