I have this concern with Charlie as well. He got to go outside almost every day off the week during summer and early fall, but now the weather just isn't agreeing with that plan. I'm in South Dakota. I have been trying to get him out for "play" in the library, but I'm noticing he's slowing down. He seems to want to play for a few minutes, but then finds a corner to go to sleep in. I'd suspect brumation behavior except that he is in a high traffic area of the library and is very popular with children and adults alike.
Can you get Dexter out and just hang out with her on the floor? I do that with Charlie. It seems to prolong the time before he goes to find a napping spot. I figure it's good for our bonding anyway.