We rented the "Picnic Shelter" on the Canadian side. It is only $200 for the whole day, and includes a fully equipped kitchen, hall with chairs and tables for around 150, one huge barbecue pit and a fireplace. (The public can still use the other huge barbecue pit and picnic tables around the site.) Book it well in advance for summer weekends, since many people use it for anniversaries, club parties, or just barbecues. Call the manager ("Contact Information") to book.
BC Government's site: http://www.env.gov.bc.ca/bcparks/explore/parkpgs/peacarch.html
Peacearchparg.org has information for both US and Canada halves of the park: http://www.peacearchpark.org/peacearch.htm
Click on each tool to see a sample.