It's generally a very good idea to clean your hot tub filters on a regular basis. Initially you're going to need a few supplies, if you don't have them already. Most of these products can be purchased at your local pool or spa shop. Regrettably your regular household cleaners are not going to do the job effectively - sure they might clean it ok, however they won't necessarily sterilize it properly which will the whole unit unhygienic. Also household cleaners produce lots of bubbles, and the last thing you need is your tub becoming a bubble bath!
If you go for the spa voucher, you would have to splash out on another gift if your loved one is simply materialistic. Alternatively these Spas usually have a Spa Shop where you are sure to find something special to surprise her post Christmas. You will appear to be very generous.