Golden Sands is mostly geared towards the sun.  However there are plenty of things to do when it's raining:

- Varna malls are big and modern and have a good selection of modern fashion and dining facilities.  Theres at least 2 huge ones: the Varna Mall and the Grand Mall.  The grand one isn't really bigger than the other one.

 - Play a few games in the casinos like hotel/casino international.  Just don't forget a basic rule: the house always wins.

- Get a massage.  A lot of the hotels on the strip offer them if your hotel does not

- Go on one of the excursions that involve 4x4 offroad activities or bus travel to nearby communities.  You would need to book such an excursion a day in advance though.  The same goes for excursions to Nessebar or Balchik, little towns that are worth a visit- while not perfect in the rain, you won't loose a day of sun if you do it while it's raining.

- Visit one of the monasteries, for example Aladja monastery, but bring an umbrella for the parts that are not inside the rock.

- Visit Varna and catch a piece theatre or other cultural activities like the archeological museum or the cathedral.

- Go karting

- Go out and eat locally, catch a performance at a local bulgarian restaurant.  A taxi driver might know a good place near you.

- Just keeping doing what you always do, chances are the rain will pass after an hour.  It seems to rarely rain all day at Golden Sands.