Should I bring a bag/purse?

I'm going to a concert in a few days with my boyfriend and his parents. The concert isn't until the evening but we're gonna be hanging out pretty much all day. He's 16, I'm 15. I'll be going to his house in the early afternoon and its going to be super hot. I don't wanna be all sweaty and gross in my concert clothes before we even get there (its outdoors) so should I bring clothes to change into before we go? Also, how can I keep my hair looking decent? If I do bring a bag/purse should I leave it at his house? Because I don't want to carry it around at the concert. But we haven't hung out too much so I don't want them to think I'm just barging in. Please help lol sorry its so long!