S.M.
Hi S.,
Wow, if you have a pool in your backyard -- I would vote for that and have someone cater the event (hopefully this would minimize the work for you).
If this truly is her last "big" birthday party, you could help her make a difference and set it up as a community service party. For example, for my kids' 10 year bdays (I only give my kids two big parties -- one at 5 and one at 10), they must choose a charity and the presents given would be provided to that charity (i.e. animal food and toys for the humane society, or clothes for a shelter, food shelf items). The friends can wrap the gifts for fun and then she does have some fun opening and delivering them (hopefully as a group to the facility).
You could also make it as a scavenger hunt -- put a list together of items needed at the food shelf. Divide the kids into teams and they scour the neighborhood and collect items on the list. Whatever team collects the most would obviously win.
Michael's Craft Store has a party room (for kids 5- 12 yrs, I believe) and it is about $30 to rent. Then the kids make a craft which ranges in price from $6 to $10/person. The facility I think holds only about 12 kids, so not sure how many kids would be involved, but it seemed inexpensive to me.
Another option is the Jump warehouses. They have about 4 inflatable slides, etc. the kids romp around the area having fun and then the cake and presents are in an adjacent room. I know a couple in Edina off 169, but one might be located closer to you in Eagan.
Let me know what you decide, since I like to hear other ideas.