The portable tables I've used are the wooden snack tray tables that come in a set of 4 for about $40 and the light weight folding picnic type tables that are about the same price for 5 or 6 ft table.
What I've found that I like even better are the school desk type tables that are about 4 ft x 18". The legs are adjustable and they are quite sturdy, even if a bit worn from being in a school classroom for however long. I found mine at a yard sale from someone who got them in bulk at auction when the school replaced them with new ones. He was selling them 4 for $30. I convinced him to let me have 2 for $15. Then went back a few weeks later and got 2 more for use elsewhere.