I know you said your mom has been ill and they don't get out as much, so you weren't certain about a restaurant GC, but I still wanted to pass on that one year my parents got my GP's a bunch of Luby's gift cards. They were NOT the sort to ever leave the house and were very much not one for spending money when they could do it for themselves, but they were tickled pink with these -- simple food, easy to get in and out.
Or if she's been ill and not able to cook much, what about stocking her freezer with one of those make-your-meal places. I know at $50/pp it will only buy about 3-4 weeks worth of main dishes, but rather then using them up every day, they can be for the days she's not up to cooking. Or if you're nearby you could give her a homemade gift card to make her X number of freezer meals yourself.
Otherwise, some things we're doing/have done for the parentals:
Dad and FIL are getting these baseball hats that have built in LED flashlights in the brim. You have to look close to even see them, just looks like a regular hat. Kind of silly, but pretty useful in a way for anything from needing to step outside the house at night to get something/look at something, for campers, people doing repairs in tight areas, etc. With these they're getting Lowes gift cards.
moveable misters for the backyard/garden (they like to sit outside and do yard work, but it gets awfully hot)
firepit for the backyard
glider/swing for the backyard (hmmm..seeing a theme here)
DVD's for old movies (Arsenic and Old Lace, Quiet Man, Philadelphia Story, etc. DH's grandfather is crazy for old westerns and they got him a box set of these).
GC to Hobby Lobby or Michaels b/c mom's into crafts/knitting.
Nice bottle of Port for DH's mom (she had told us that every night before bed she has a small glass of port and as a result sleeps like a baby

)
One of those emergency radios/flashlight that is powered by a crank -- maybe not a an issue where you are, but really handy for my parents being in tornado alley and subject to a lot of power outages. I also considered getting them these cool rechargeable flashlights. You plug them into sockets around the house where they function as nightlight, but when the power goes out, you unplug them and they turn into flashlights.
High Tea or a play -- one year my sister took me and my grandmother to a tea as her christmas gift to us. We had a wonderful time

. Alternately, you could take one or both of them to a holiday play (once took my mom to A Tuna Christmas -- hilarious!) or buy tickets for you to take her to a play later in the season. They get the gift of time with you and a night out.
My mom doesn't cook as much anymore, but she still likes cookbooks/to watch Food Network, so I've alternately given her: Paula Deen's cookbooks, Alton Brown cookbook, and this year she's getting the new America's Test Kitchen cookbook with all of their recipes.
Police Scanner -- though you may have to figure out how to program it for them. We got one for dad thinking he'd get the most out of it, but even mom snoops with it, lol!
Good luck to you, I know it can be challenging!