Are these interior or exterior security bars on lower windows ? If so, you may not be permitted to obscure them, for safety reasons, in event of fire.
If it's a matter of setting plants on interior window sill, you have many options, but I would not train plants to grow on window bars.
use a trellis in front of windows on inside or outside then use twist ties,floral wire or zip ties to train or just keep plants on the trellis. You can put the trellis(small/tall/slender) inside the pot/container plant is in just stick it in the dirt carefully don't damage roots. You can make your own using string or wire and thumbtacks running from bottom of plants/pot(tie string around top edge of pot so you have something to attach strings/wire to if no trellis) to upper or sides of window edging. if its metal framed window use either tension rod across window or command hooks. Set pot in center of window sill/on floor/or hang above from ceiling then direct plant stalks/stems upward or downward & outward in fan shape to get most coverage across window. To get most types of plants to produce flowers you'll need a sunny window or at least very bright light.
Hello. You could put pots or install window boxes. Then plant vines. Once they start growing train them to grow on the bars. Good luck.
You can weave artificial ivy around them , adding some silk flowers. Or lay a shallow planter ,you can use several drawer storage, like for pencils . And real plants in them . Lay a single layer of small pebble or rock on the bottom for drainage.
Plant a vining plant that also flowers. You could put a small trellis in from of the bars or just use the bars as a trellis depending on their design.