M.C.
A.,
My youngest son sometimes goes to someone's house (they have their own mini bounce house). They just grill out and the kids just play and have fun. Most of the time the end of season soccer parties are held at Gatti Town in Frisco. It is a popular place for end of season parties. I would e-mail some of the other parents and see what they'd prefer. For me, Gatti Town got to be a bit pricey when I had 3 boys in soccer and their parties were at different times. It costs just to get in so there really isn't an option of only paying for the child. I also prefer to just be at someone's house because the parents get to visit (instead of chasing the kids around the game room) and the kids just play.
I don't know if you have anyone taking up a collection for a small gift for the coach, but that is a common practice as well. Everyone usually pitches in $5 or $10 and then the one in charge usually gets a gift certificate to somewhere.
Hope that helps some!