I think it would be fun to have the guests "get creative" with their diapers. We have all seen diaper cakes, but it would be fun to see what other "sculptures" people can come up with to make with diapers!
I realize that people might not be as creative, but that is what popped into my mind as I was reading your post...
At my shower we did the candy bar in a diaper guessing game - "What did baby eat"...melt a different candy bar (reeses, twix, crunch, etc.) in each diaper and guests try to determine which candy bar is in which diaper. Probably the only time people might be willing to sample the contents of a diaper!! :-)
Another activity I just did at a friend's shower was writing notes on diapers. Guests wrote notes, drew pictures, etc on diapers so parents would have something to look forward to during diaper changes. Some people were encouraging, some funny, and some just creative/artistic! It would be fun anticipating what would be on the next diaper you pull out! :-)
Definitely encourage different sizes of diapers - you could have different drawings for different size diapers - everyone who brought size 1 in one drawing, everyone who brings size 2 in another drawing...
Or for every pack of diapers/wipes you bring you get a raffle ticket/entered into a drawing. That is always a fun incentive to bring diapers.
Have fun! :-)