Re: Things to do in a mehndi - other than sing/dance/play music
I've been to many mehindis where there is no music and dancing. If it's all females you can sing or dance if you would like. You can also play games. Hmmm... another beneficial thing i've seen at many mehndis is married women, (especially the mom, aunts, grandmothers) give advice to the girl who is getting married, sometimes funny little dos/don'ts. There is a very good hadiths or athar about the advice a mother gives her daughter, if you want I can find it and post it for you.
Just have fun with your friends and family... everyone can get mehinde, talk about what they'll wear for shaddi, gossip about your dude ;) and eat yummy snacks :-)