Now that the weather has cooled off (and HOW!), my cats at home have become much more active and playful. Boochi has gone back to one of his favorite pastimes: yelling and bopping around my home with tail puffed, flinging toys hither and yon, and calling Hobie in an effort to get Fatty Himself to play. Dusty has even perked up a little and started batting her mousie "babies" around under my desk in the office, though at 18 1/2 her play sessions are remarkably brief.
Water consumption is down, and food consumption is up. When it's ghastly hot outside, most cats do a lot more drinking than eating, even with air conditioning on.
So if you are noticing these things, don't be alarmed. It's all normal for this time of year. Of course, if your cat is way too active for its age, or eating a whole lot while losing weight, be sure to make an appointment for an exam as a serious medical condition could be developing.