A trip to a plumbing supply outlet, a few dollars and a hour or so later gives you a practical, waterproof and cheap solution to your picnic table canopy needs. Screw a PVC flange to each end of the table, insert a matching length of pipe in each, and top those with a PVC tee. Add short sections of pipe to the ends of each tee and use curtain rings over the pipes to secure a cloth in place between the new tabletop T-posts.
This arch enemy is not yours but the sun’s -- making it your new best friend. By drilling four small-diameter holes in the four corners of the picnic tabletop and tapping in a corresponding diameter dowel 3 or 4 inches long, you have the beginnings of an arched canopy you can adjust to block the sun. Slide a length of PVC pipe over a dowel on one side and arch it across to the dowel directly opposite to hold it. Repeat for the other end, then fasten a cloth between the two arches using either round curtain rings or hems in the material to gather the cloth on the rods. Slide the cloth across the arches to block the sun as it moves.
Sink four poles into the ground around the picnic table to stretch a tarpaulin or fabric between, and use small ropes anchored between poles and tent pegs to hold the poles rigidly upright. This is a good way to create quick shade without damaging the table. If the ground is too hard or you prefer not to dig holes in the lawn, place pole ends into holes in concrete blocks and fill around them with gravel to anchor. Disguise the concrete blocks with a potted plant set in the unused hole of the block.
If you have them, trees make exceptional canopy supports. Even a small-diameter tree of sufficient height is sturdier than a pole in the ground. Tie a rope between two handy trees and drape a cloth across it. Use lighter-weight twine to fasten the corners of the cloth out away from the table to tent pegs pounded into the ground for a simple and elegant tent canopy. A bamboo screen makes an excellent, natural alternative to a sheet as well.