Welcome to the Bunnings Workshop community @herno. It's fantastic to have you join us and many thanks for your question.
To rejuvenate cedar you would start by sanding all surfaces until the weathered look is removed and it returns to its natural appearance. You'll note a cedar smell once you get it back to fresh timber. You can start with 60-grit sandpaper and progress through 100-grit and 180-grit until the finish is completely smooth and scratches, splinters or gouges have been removed.
Once the timber has been sanded there are a few schools of thought about what should be done next. Some recommend not applying any coating at all as cedar has a natural resistance to rot and decay. You can apply linseed oil to slow down the aging effects and extend the period in which the cedar needs to be sanded. Alternatively, you can apply a hard coating similar to Cabot's exterior clear polyurethane.
I would advise sanding back the cedar to restore the natural look of it and applying the Cabot's product as it is a durable marine-grade polyurethane that provides protection and shine without hiding the grain. I wouldn't leave it un-sealed as it is next to a spa and linseed oil is only a temporary solution.
Please let me know if you need further information or have any questions.
I am lead to believe that if the cedar is not treated, to clean it you should use a citrus based cleaner. If you want to make a natural citrus cleaner that can be used anywhere:
8 oz of orange / lemon (peel only the peel)
a bottle of white vinegar around 16oz
grab a container place the peel in and pour the vinegar in, ensure peel is covered completely, screw on a lid
Let stand for around 5 days
pour into a spry bottle and give it a go
Not sure of this as I have not used it for cleaning timber Simple Green (citrus type) however I use it for cleaning sanding belts and sanding pads Bunnings has it have a look
My money would be spent on a clear marine Polyurethane too. Go for a couple of coats with a light sand with fine sandpaper in between coats. That will give you deep looking finish.