|Fruit in our water bottle? What an intriguing idea!|
Guinea pig can have unsweetened apple juice and unsweetened cranberry juice. We really need to emphasize the unsweetened part because fruit juices are already naturally sweet, and you don't want to give your piggy too much sugar. In fact, even unsweetened juices need to be diluted before you let them drink it. Their bottle should have no more than half juice, and the rest should be clean water.
You should let them have this no more than once a week because of the sugar, even when watered down like this. Make sure you give your piggies less fruit if you're going to give them juice that week. In addition, you should refrigerate any remaining juice overnight at the end of the first day, and discard any remaining juice after the second day to avoid bacteria.
Cranberry juice may actually help prevent urinary tract infections, so it's probably the better choice if your guinea pig likes it (not to mention the recent study finding trace amounts of arsenic in many brands of apple juice!). Just make sure the label says 100% cranberry juice on it. If it just says "100% juice", it may be juice from a variety of different fruits, and if it uses words like fruit "beverage," "drink," or "cocktail," it's probably not 100% juice.
Even with this in mind, deciphering labels can be complicated, so you should probably buy the juice from a health food store like Whole Foods to be on the safe side. Lakewood Organic appears to be a good brand.