You might even want to do the battle scenes with Beowulf and Grendel, which are exciting and interesting for an audience to watch (and fun for the boys, for sure, to do) and you act as a narrator or reader of the action. Of course you can also be part of the actiing, and I wouldn't worry at all about making sure you have a female role. There just aren't that many females in the story, so if you want to act just choose something you all really like and don't be trapped by gender. In fact, for fun you can have one of the boys play a female role--if you choose a selection which has a female--just to make the point that it's about plot and character, not gender. Have fun!
I would start by finding the part of the book that you find most interesting or that might end up being the most compelling in a five minute production. Once you've found this, you can go about choosing characters and building from there.
You might want to find a part of the story where you can involve Grendel's mother since she is the only female with a major part, but you can obviously change roles as you wish and needn't be bound by gender either.
You could also choose two different parts, perhaps Beowulf's battle with Grendel and then the final part of the book where he fights as an old man and is defeated but dies happily.
If I had the grouping that you have I would do a part from the play when Beowulf and his men are before the king. The queen would also be present. It is from episode 6 when the queen speaks to Beowulf.
You could have the following characters: Queen, Hrothgar, Beowulf, Wealhtheow, and the two brothers. (You can do it with only one brother).
The site that I listed includes the English version of Beowulf which will help you to understand it a little better.
The scene takes place in the great hall where there is music and entertainment.