The phrase "am up and about" generally implies being active or fully functional. Its antonyms would be terms that signify lethargy, inactiveness, or temporary inability to move around easily. Some of these antonyms are bedridden, incapacitated, immobilized, inactive, lethargic, low-energy, passive, sluggish, and weak.
For instance, someone might say, "I was up and about all day, running errands and getting things done." Conversely, they could say, "I'm feeling lethargic and not up to doing anything today." The antonyms of the phrase can be useful when discussing physical or mental impairments, taking sick leave from work, or generally feeling less active than usual.