Mostly water as it helps replace fluid lost in sweat. While working out, you should be drinking 1 cup every hour unless you are sweating in a hot environment. Then you need to drink more and may need to replace with a beverage that has a small amount of sodium and potassium.
One way to tell if you are drinking enough water is to weigh yourself before working out, drink water during your workout, then weigh yourself after your workout before urinating. For every 1 pound of weight loss, you should drink an additional 2 cups of water.
Your next meal should have some carbohydrates (pasta, rice, potatoes, starchy veggies, fruit, dairy). This helps replenish your blood sugar and glycogen stored in muscles IF you were doing exercises of short duration like most sports other than running. If you were doing aerobic exercises and taking in enough oxygen in order to carry on a conversation, you shouldn’t need more than a moderate serving of carbs at the next meal. Your body switches from burning your blood glucose and muscle glycogen after 20 minutes of exercise to burning body fat.
If you were weight lifting and are not competing in a sport that needs a larger muscle mass like football or bodybuilding, then there should be enough protein (3 – 4 ounces of meat, poultry, fish, eggs, cheese or soybeans (tofu)) in the next meal.