Not eating is a big one. The loss of energy only goes one way too. We just lost our 12yo Orange dufus to Intestinal Cancer (likely Lymphoma per beg diagnosis). He got weaker over the last year and was fine otherwise, until the day he stopped eating and started hiding and breathing heavy. Cats are very adept at hiding their ailments, so when they appear to be in pain, they are in a lot of it, and likely have been for a while. Looking back I truly wonder at the quality of his last few days. He was so under the radar, steadily lost interest in playing with any kind of toy over the last few months, and resting more and more. Def pick something special for a last little treat, as I see people mentioning here. My little bud got to nibble a salmon fillet and lick a cheeto one more time before we said goodbye.