Lucky for you ladies, San Francisco is very walkable and public transportation-friendly, so you can get around for fairly cheap!

Here are my list of favorite budget-friendly places:

• Ferry Building: Try to come here on your Saturday morning in SF because that's when the famous farmer's market is!  It's not just for buying fresh, local produce.  There are also a ton of great gourmet food stands (a lot of which have actual restaurants), so you can experience the awesomeness of San Francisco food.  Inside the Ferry Building itself is also great to experience.  If you like cheese, got to Cowgirl Creamery And if you like coffee, you have to go to Blue Bottle Coffee
• Cupid's Span: While at the Ferry Building, walk over to Cupid's Span for a great photo op of the sculpture and the Bay Bridge.
• Waterbar: If you like oysters at all, walk a little further down to Waterbar.  It's a gorgeous restaurant with a beautiful view of the Bay Bridge and the water.  It's a pricy restaurant, but they actually have an awesome happy hour menu.  From 11:30 AM to 5:30 PM every day, they have $1 oysters!  Try to go earlier in the day though, because they don't take reservations and they get packed.
• Palomino Restaurant: If Waterbar is packed, go across the street to Palomino.  Around lunch time is when the sun hits their patio the most, so try to go there then.  Another gorgeous restaurant that looks pricy, but actually has an awesome all-day happy hour menu.
• Justin Herman Plaza: If you walk across from the Ferry Building to the right, you'll see the Justin Herman Plaza.  The egg head sculptures are fun to photograph and there's an amazing industrial-looking waterfall structure that you can actually climb and also walk through (just discovered this the other weekend!).
• Golden Gate Bridge: Something else that's fun to do is to rent a bike and bike across the Golden Gate Bridge.  You can also walk across it.  It's typically an all-day thing, though, if you can allot the time for it.
• Golden Gate ParkThere are tons to photograph here because it's a huge park! De Young Museum is beautiful on the outside even if you decide not to go in. Stow Lake is gorgeous, as are the Japanese Tea Garden and the San Francisco Botanical Garden.  The outside of the botanical garden is beautiful, and if you want to go inside, it's only $7.
• If you want to go barhopping, the Mission District and Polk Street are great streets to walk through.
  
• For a fun place to picnic, you have to go to Dolores Park midday to picnic!  It's the thing to do!  Grab a bottle of wine Bi-Rite Market, maybe some ice cream at the Bi-Rite Market, and a sandwich from Ike's Place or a burrito from El Farolito and just laze in the sun.
  

Have fun, ladies!!

For the outdoors on a budget, please do not leave without walking through the Muir Woods. There is a shuttle service there that is inexpensive. Visit here for more info: http://www.parksconservancy.org/  If you have never experienced redwood trees, it's a must. 

If you've never been to San Francisco before, and are open to something new,  AsiaSF can be a fun girls night out. Music, good food and dancing after dinner, if you're in the mood. 

Drive up to Napa/Sonoma, get a couple of amazing sandwiches and other picnic treats at  Oakville Grocery, and do some wine tasting.  

See if there's Shakespeare in the Park playing at  Golden Gate Park, or take in the Aquarium or  De Young Museum.

Have fun!

Obvious answer is Golden Gate Park, Golden Gate Bridge, Ocean Beach, Crissy Field picnic

Twin Peaks in the night - I would recommend and hope it's not foggy 

Fisherman's Wharf, Ghirardelli Square Shopping Center, Alcatraz are all fun places to spend time in SF and relatively low cost.