First I would start with discussing ideas with the bride, I know it's for the bridesmaids to plan but it is for the bride and if you have an idea of things she likes it makes it easier to plan and this will give you a base to start with. Ex. find out if she wants games played; some people despise though shower games, while other people just absolutely love playing them.
Once you have a base you can plan for food and drinks. Make a list and spilt it amongst the bridesmaids and mothers (if they are willing to help). You can pick a theme for food or you can just go with appetizers and desserts or add a main dish. Drinks can be punch with and without alcohol, wine, soda, water, etc. It's good to have a variety if you don't know what the crowd is going to be like.
Games; Crystallee mentioned some good games. Since it's going to be a large group you are going to want to keep the games simple and a max of 3 games! Another suggestion is Bridal Bingo! The guest get a blank bingo card and have a choice to fill in the blanks with an item the bride to be may get as a gift. When the bride to be opens her gifts and each time someone has that item on there card it gets marked off until you get bingo. I think this is a great game to get people more engaged in the gift opening part of the shower.
I also played the 'how well do I know DH' game at my bridal shower and each person read off a question when I opened the gift they got me and if I got the answer wrong I had to put a piece of bubblegum in my mouth
Another game is finding fun stuff to put in the bride's to be purse (this can be played to different ways) and have the guest guess what's in the purse; the person with the most correct answers wins, or you can have this purse knocked over and the bride to be with pick up everything as quickly as possible and afterwards the guests have to write down what they saw from memory. The fun part is, you can put goofy stuff in the purse to embarrass the be bride to be

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Either way, I'm sure you will figure something out and it will be tons off fun! Just get with your bridesmaids and get there input as well.
I have thrown quite a few bridal showers and recently baby showers so I have theses things down LOL.