aaaaahhhh...i SO know where you are right now. my allergies are AWFUL! i have had them since about age 2 and several years before having children, i found the ONLY thing that worked was a one week course of oral steroids. now that i've been pregnant or nursing since 2004, my options are limited. it sucks! so, you get tons of sympathy from me. i know it sucks and i know how it feels to live in some kind of fog of antihistamines and decongestants.
problem is, i have little to no advice! yesterday, i crashed on the couch after lunch with a wet washcloth over my eyes. ds2 was napping and ds1 (3.5 yrs) was wanting to play. luckily, he was happy to play doctor on me and i got rubberband hospital name bands on my wrists and proceeded to have multiple head to toe exams and some bandaging and shots. it worked for a good half hour. at least it was a little rest.
i pretty much have to refuse outdoor play but i let my son play on his own outside - right near the front door where i can see him - this may not be an option for you though???
i had a little luck explaining my "sickness" to ds just so he'd understand that i don't want to feel this way and that i don't like not being able to play - reminded him what he felt like the last time he was sick. i think he got it and was a little more "content" to play alone. i sat right there with him, but didn't get involved being talking.
maybe get/make some new "cheap" stuff and stash away for when it's really bad - then you could whip out some fresh crayons and paper/books, new playdough with some accessories, some building tools like tinker toys, cups in a big pan of dried beans (measure, pour, dump), some stickers and paper, etc...
good luck!

: