Sure you can go to the beach on prednisone, just be careful, as everyone has already suggested.
Some meds don't do well with sun exposure, sometimes they put a warning on the pill bottle, sometimes there is no room for it.
One I am aware of is Phenergen. It makes you tan/burn MUCH faster, so you have to be careful, and wear lots of sunscreen.
You can look up all your meds, and ask your pharmacist too.
Phenergen;
increased sensitivity to the sun or ultraviolet light
Keep out of the sun, or wear protective clothing outdoors and use a sunscreen. Do not use sun lamps or sun tanning beds or booths...