You really don't need a trellis for anything. Stuff can just spill over the sides of the planter with no issue. If the planter is raised, the parts of the plant hanging down can let you imagine it's on a trellis. It isn't as if the parts of the plant laying on the ground are going to be sitting on soil. But what you try to grow is going to depend a lot on how big the planter is. That being said, plants that try to climb (pole beans, peas, etc.) might not be smart. If your planter isn't that large, I'd stay away from veggies with large fruit, like melons, squash, cukes, large tomatoes, etc. Cherry tomatoes could be a fave. They'll grow just about anywhere, and you don't have to be concerned about attracting pollinators, who might not have any reason to be up on a rooftop. Same with small peppers.

There are a lot of new tomato varieties specifically bred for container / patio gardens. Some even grow in sort of a bush-like form that do not require a trellis. Peppers don't really require a trellis. I've also grown patti-pan squash in a container. I just used an obelisk in the container. It holds itself up.

You could also use tomato cages as supports. They don't require being secured elsewhere just anchored in the pot by the soil.

The full range of greens, peppers, eggplant, dertiminate tomatoes, potatoes, most of the root crops, celery and celeriac, bulb fennel, kohlrabi, summer squash, bush beans and short peas, bush cukes, lots of different herbs, ginger, turmeric, alliums... and there are several edible flowers, including my favorite nasturtium. Jerusalem artichoke, sunchokes, and sweet potatoes too. I'm probably forgetting some, heh.

All depends on the size of your planter. My container gardening tends to range from 5 gallon to 25 gallon containers, though I like the 18 gallon plastic totes a lot. If your planter is shallow, watch it on some of the root veggies. Though not needing a trellis, some veggies can benefit from support like tomato cages.

Lettuces and Asian greens like tatsoi and mustards are super easy to grow in containers and pretty fast too. I grow green onions in containers too, along with basil and mint and other herbs.

If you are sowing seeds, Pinetree specializes in smaller varieties for container gardening and small quantities of seeds for home gardeners who don't need a large amount. I've been growing their lettuce mixes and other greens for many years now...

superseeds.com