Honestly?

[manuel]

This will be kinda hard to top next year. 

[MaxXimus]

I'm sure something can be pulled off

[MS-DOS4]

This year, I swapped everybody's names, avatars, and signatures on my forum. It was moderately entertaining for a while. Perhaps an idea for next year?  

edit: actually, probably not a good idea on a forum that conducts business...

[manuel]

Yeah, we'll think of something. 

[P]

Although I thought it was an obvious April fools it was still funny!

[manuel]

It will always be obvious, because most people go into not-wanna-get-fooled mode by March 31st and doubt everything. 

[P]

Yeah but then you let your guard down and bang!

The biggest reason regulars are not easily fooled by this is because they know you always do something funny with the site on April 1st so when they see something strange they immediately check the date.