I'm going to parrot a few of the ideas, but get a little more specific.
And depending on how much lead time you have.
Do you know what series your father prefers?
No matter what, you're safe with a yellow, red, and blue color theme. Decorate balloons and cups with either regular or metallic markers, or paint. I would put the classic delta logo on cups and balloons and I love the Trek quote idea, those might be cool on the cups too, as conversation starters/trivia questions. "Who said..." You should be able to find Trek trivia questions online.
You should also be able to print out some related art to get ideas, just do a "Star Trek" google images search, take a look at the fan art forum here, and you should come up with more ideas than you'll ever need. I would also go to StarTrek.com. for pictures and ideas.
I would suggest you mock up a few console-like control panels, again you can find those and either print them out, or make some with markers on foam-core board. You could also make PADDs, ship silhouettes, phasers and tricorders out of foam-core or craft foam (depending on his favorite series). If you don't know what phasers and tricorders are, just google them, and you'll get lots of pictures.
While you might want to go extra fancy and make him a quality "cosplay" uniform of his favorite series, you can go pretty low cost for your "extras" by using long sleeve, t-shirts or mock neck, and decorate them with airbrush shirt paints, and fabric paint for the delta/comm badge.
The original Trek (Captain Kirk) would be the easiest, (Gold, Red or Blue shirts, standard collar painted black, gold ric-rac around the cuffs and black and gold shirt paint for the delta) Here: http://www.st-spike.org/pages/unifor...0/uniforms.htm
If you know someone who does T-shirt airbrushing, then TNG should be pretty easy. A long sleeve, mock neck shirt, with gold and silver fabric paint for rank pips and comm badges.
Class A uniforms for TNG (Picard) I'd use a Gold, Red or Blue shirt with air brushing to add blacks on the shoulders and waist.
Class B uniforms for DS9 (Sisko) & VOY (Janeway), will be trickier because it's so black heavy, not sure what to suggest there.
ENT (Archer) period would probably be the 2nd easiest, I'd just go with blue long sleeve t-shirts with standard collars, and deco them with airbrush/shirt paints.
If he's a general Trekkie than you could do a variety uniforms.
Hope that helps.