Alright here are some idea:
Balloons with ST language on them. Either store bought or Google famous ST QUOTES & USE A SHARPIE!
Buy some CHEAP ST uniforms or have T-shirts with logos/graphics
Have CD with ST music/Remixed songs-one GREAT
choice is the pop group Information Society!
Have tribble props lying about
Do a little minor make up for your guests to make them "alien"
Guest could have Communicator / phasers props.
Birthday Games-ST Theme or an actual Costume contest.
Birthday Cake ST theme
If you like take Family Photos and Photo Shop them to put Family into ST setting / costumes or even scenes & run a slideshow for the enjoyment of all
Lastly...invite me LOL!