I suspect it is better to sit in the sun without exerting yourself. Save your muscles for pumping up.
Cardio should excrete some subcuteneous water and sodium through sweat, but will also use muscle potassium and muscle glycogen.
Sitting in the sun will dry you up and secrete sodium without deflating your muscles.
I guess a very small amount of cardio, very brief, could do something... I don't know what though.
By the way I have read that Negrita Jayde used arginine to pump-up and harden in her pre-contest diet. Probably won't help much if the photo shoot is really soon, though.