Clever Baby Announcement Ideas to Surprise Parents?

Ok...a little back story/info.  I am extremely close to my parents who live 7 hours away.  I am pregnant and so excited but have not told them yet! They are very close to my 3 year old son and my 2 step-sons (ages 8 and 11), and we last saw them about 4 weeks ago.  I was not showing then and did not say anything because my dad was in poor health, and I didn't want to worry my mother since I have had 2 miscarriages in the past 3 years.  Everything is going smoothly, and I will be 14 weeks when I see them on Memorial Day weekend for a family camping trip.  I have an ultrasound the day before we leave, so I should have a good pic. Also, something fun...I AM HUGE! Ive read that your uterus expands quickly after it has already stretched, but this is shocking LOL! Im petite, so there really isnt a way to hide this from them any longer.  My sister in law knows, and will be meeting us there, so she can help with any necessary set up.  I'd like to incorporate the other 3 boys somehow...Any ideas?
All I could think of is that my 3 yr old (who they always get out of the car first) should wear a shirt that says "Mommy Got Fat"
Next boy's shirt "But dont worry, Dad says she will lose the weight soon"
Third boy shirt "Yeah like in 5 to 6 months."
Then I'll waddle out of the car.
Cute? 
Im always scared to go with first idea, but what else can I do to surprise them as im getting out of the car? As soon as they see me they will know!
No ideas are bad ladies...get creative and Ill send ya'll pics!
Thanks!