Not sure I can quite picture either the item (she sits on a ball?) or the occasion (you're leaving and she gets the memento?), but just skimming through some phrases, what about

For someone who's always on the ball (= ready, well informed, up to date)
For someone who's a ball of fire at work (= fast, vigorous, enthusiastic)
Always keep your eye on the ball (= pay attention)
Good luck keeping the ball rolling (= keeping everything going, not stopping)
For someone who's on a roll (= consistently lucky)
Hope I haven't left you rolling in the aisles (= laughing uncontrollably)
Hang in there and roll with the punches (= survive, adapt to bad luck)

A seating ball looks like this...

http://www.postureprince.com/balls.html

These balls are good for one's back. It is recommended not to sit on them all day but switch between the ball and a chair regularly (using only the ball is too much strain for the muscles).

I think the suggestion 'for someone who's always on the ball' is perfect considering that in this case she is literally sitting on a ball, as well as it being a compliament to her working skills!

Just think: You'll still be here while I'm having a ball somewhere else!

("Having a ball" means enjoying yourself very much.)

Ok, ok...I'll think of some rude ones since the rest were already said.

Balls!
Got balled lately?
Roll this in flour to find the wet-spot
....

"You Sure Love To Ball" - great song by Marvin Gaye

How about ...

"What do you keep in here?"

And if you want a rude one ...

"Does it feel nice if I stroke it?"

or see "Ballbreaker" thread....! On 2nd thoughts, maybe not!

you boys! Finbar Saunderses the lot of you...

in Anlehnung an die weebles könntest du auch schreiben "XY (the woman) might wobble, but she won't fall down".

But I also like the idea of something being inside it - you could write SWAG in large capital letters on it...