Make fun of yourself. Self-deprecation goes a long way, especially if it isn't serious. While being too hard on yourself can be a sign of weakness, when you're telling a girl to come over, you can show your funny side while sending your message. Tell her you have a jar of pickles you aren't strong enough to open and you need her help.
Explain that you don't want your date or time together to end just yet. Depending on the situation, there are other opportunities to be sweet. For instance, offer to make her dessert, have a drink or pop in a movie, depending on the situation.
Tell her you're having trouble sleeping without her next to you. If you've never spent the night with her that won't work, so ask her to come tell you a story to help you get to sleep. Even if you aren't having trouble sleeping and she knows it, you are being original and using your brain to signal that you want her to come over. Women notice these things and will respond, if they like you to begin with.
Offer to help her with any work she has to do to help her prep for the next day. If she hasn't been getting enough sleep or has been stressed, this shows you pay attention and are willing to help make her life easier.
Make her something and tell her you have a gift for her. If she likes you and you have spent time together before, this probably will be enough to entice her to come over, even if she has other plans. You don't have to spend money to give gifts. Sing a song, draw a picture, write a story or cook something. Use your skills and talents to show her you care.