My grandma gave my mother the best 40th birthday I've ever seen. My grandma and her twin sister gave my mother a surprise 40th; I was 20 at the time, have 3 sisters. My dad said he was taking my mother out for dinner but wanted to stop by the park first to go down memory lane with her... My crazy great aunt dressed up as a big baby, had a sheet between her legs that looked like a huge diaper with pins on each side, a baby bonnet hat, booties on her feet...My grandmother and her twin met my mom with a wheelchair and a walker, they had about 10 gag gifts for her, big note pad so she could write down thing so she wouldn't forget, box for pills, just a whole bunch of stuff, we each had to give her something and make it funny... she, we, all laugh so hard...
Anyway, don't know if that helps or not, but I know my mother enjoyed it so much, I don't remember her being upset about turning 40, now in 5 years I will be... time flies...
I like the spa idea too but a few gag gifts can really make it fun... I know the brand new car would be NICE; that mama has an incredible husband to do that...