It depends.

• They could just be being social
• They might actually want to know what you're doing later
• She's using it as an excuse so that YOU can now chat her up
• She wants to know where your going, maybe to see if she's also heading there or if she can change her plans so she also ends up there

For example: "What are you doing later...." you could ask her to come to a club/bar/party. Back to yours for sex? You & I could go to place x and get drinks

The jokes thing is cool as long as there's laughter, humour or confidence behind it when you deliver it; it can backfire if you deliver it the wrong way & come off like an asshole.

If some one asks you this whether its male or female they have a level of interest in you & they are making an effort to show that interest. If a girl you like asks you this - use it as your opportunity to move things along with her. If you're not overly interested, then be honest & tell her you're going home to sleep or whateevr etc.