In some ways, a party without gifts, if like a party without the birthday cake. As singing Happy Birthday is always a treat, and blowing out the candles!
Reading that some of you don't even open the gifts, at the party until later in the week?? My child would love to see their friends reaction when they open the gift. Watching all the children surrounding my child, as everyone is saying, "open this...!"
I never expect an expensive gift.
I also never invite the whole class either!! Who seriously invites 20 kids!!
WOW... I only invite 5-8... For DD6, she had 6 kids...
Several times, we went to a hotel and my child could have ONE child sleep over, but a few come for the afternoon. This is my gift to my child...a hotel. Of course I invite family over, AFTER the party, to enjoy time with us too.
My parties usually last 3 - 4 hrs. When a party is only 2 hrs, I feel the parent is just trying to get it over as soon as possible! Can't see any child wanting such a short party. However.....however... I don't blame the parent if they are foolish enough to have 20 children!!!! 
I think the Loonie/Toonie/Book Party are really neat ideas and will try that!
If your invited to a party, no gifts required, then I guess you respect that...