I hope your not flying back to where ever your from
@kkt3 .
The bull kelp and other kelps you can dry them out and crush, etc but they are pretty big. You can just cut them up, throw em in a blender and toss in with your soil. Let it cook (break down) for awhile etc and use it when its your next go.
Honestly, if you live here its easier.... I wouldnt bother taking kelp and other large things back with you. You can buy kelp and other sea goodies ready to use at your local hydro store or on line. I'd go that route. Consider just taking easy things like sands, soils, and shells etc. Way easier and less hassle Plus no worries of stinking up the car on the way home or getting in shit at the airport.
If your from the states, I wouldnt even try taking anything home.
Its been awhile since I was into soils, when you dont use it you start to loose it, I wish I had more helpful information but there should be a tonne of info about this online to investigate what you'd want to look for and take back.