Vegetables to try:

Now through August is generally the best time to plant most vegetables. You want to have mature plants growing before cold weather sets in.

Arugula (can sow through late autumn, easy to grow)

Beans, pole or bush (pole beans utilize space better, needs a greenhouse on the warm side--You won't get as big a crop as in summer, but what you get will taste great!)

Learn to enjoy LEAVES.
It often takes a lot of heat and light to get a plant to flower and fruit.

Broccoli (needs to be planted in deep container or directly in ground inside greenhouse--heavy feeder)

Brussels Sprouts

Cabbage

Carrots (use shorter types in containers--I've had success with the planet/round types in containers as shallow as 6 inches)

Cauliflower (also needs deep container--heavy feeder)

If you are willing to heat your greenhouse to a minimum temperature of at least 50 degrees Fahrenheit, you will be able to extend your harvest or make it more bountiful.
Also, try bottom heat, rather than heating the air of your greenhouse--
but not too much, as these are cool weather crops for the most part.

Collards, Kale & Mustard Greens

Corn Salad

Kohlrabi, Turnips & Radishes (for autumn harvest only--don't do well in poor light)

Lettuce (loose leaf varieties are best--avoid types that need to form a head before harvest, may need to feed often with weak solution as roots are shallow)

Remember: seed germination generally requires heat. The plants that result will tolerate cooler temperatures.

Mustard Greens

Oriental Greens (all types)

Peas, edible pod types (must flower and fruit, so be patient and enjoy the pods you get)

Peppergrass (very quick to mature, sow anytime, easy to grow, also grow as sprouts)

Spinach (may be able to plant this through mid October, as it germinates at a lower temperature than many seeds)

Watercress (likes lots of water and bright light)

 Flowers to try:

First, look around your outdoor garden. If you have plants that are blooming in autumn, try digging them and potting them up. That will give you a bit of a head start on brightening up your greenhouse. The tenderest perennials are good candidates for this, as they'll die if you don't give them shelter over a cold winter. (Be careful that you are not introducing pests into the greenhouse when doing this. To be safe, I would give them a couple of ultra-fine horticultural oil treatments before moving them in.)

Another possibility: Take cuttings from your favorite plants. Try anything--it might work. (Same pest warning applies as above.) This is also good "insurance" against a very hard winter in which you might lose your outdoor plants. You will always have the cuttings.

Bring those Pelargoniums (Geraniums) in. Don't keep them soggy, watch for fungus problems, remove any bad leaves, and don't let the greenhouse temperature go below 55 degrees Fahrenheit. They will bloom in late winter, and you can repot them to enjoy on your terrace next summer.

Generally, blooms in winter are the result of forcing spring flowers or delaying summer and autumn blooming plants.

Abutilon (Flowering Maple, perennial, easier to start with a plant or cutting--may not bloom the first year)

Ageratum (annual)

Begonia semperflorens (Wax-Leaved Begonias. Seeds are extremely fine. In August, sprinkle seeds on surface of moist vermiculite and cover with plastic to keep moist. As seedlings grow, keep potting up and pinching. They'll bloom from midwinter to spring. Keep greenhouse temperature above 50 degrees Fahrenheit to keep them flowering.

Bellis perennis (English Daisies, perennial--can even take temperatures as low as 40 degrees Fahrenheit, put several in a pot)

Bulbs, for forcing. (If you want to do this, just buy some bulbs of the flowers you like, and trust that I'll soon help you with what to do. I will, Iwill. If freesias appeal to you, check the freesia page.)

Calceolaria (annual, needs good air circulation--watch leaf color and feed as necessary)

Calendula (annual)

Centaurea cyanus (Bachelor's Buttons, annual--grow tall types in deep tub, dwarf types in regular 12 inch pot, well-draining soil)

Don't overdo the nitrogen when feeding, or your plants won't bloom. They need phosphorous. Work some bonemeal into the potting soil.

Dianthus carophyllus (perennial, will need staking and some extra fertilizer during early rapid growth)

Eschscholtzia californica (California Poppy, annual--sow NOW or you may be too late)

Remember: seed germination requires heat. The plants that result will tolerate cooler temperatures.

Exacum affine (German Violet, Persian Violet, perennial in greenhouse, grow in 4-5 inch pot initially, likes moisture and moderate fertilizer)

Gypsophila elegans (annual--won't bloom in rich soil)

Impatiens (annual--take cuttings from your garden, but watch out for pests!)

Lathyrus odoratus (Sweet Peas--soak seed in warm water for 24 hours before planting, and be sure they are buried in potting soil as they need darkness to germinate. Don't like transplanting, so use peat pots or pot in container they will live in. Fragrant. Get a variety bred especially for winter growing such as 'Winter Elegance.'

Growth won't be as rapid, but the eventual flowers will last longer.

Lobularia maritima (Alyssum--annual, fragrant)

Matthiola incana, M. bicornis (Stock--fragrant, grow larger varieties in deep container, dwarf in regular pots)

Myosotis (Forget-Me-Nots, annual, start in early August, need good air circulation)

Annual plants will only bloom once. When they're finished, toss them on the compost pile.

Nasturtium (annual, doesn't transplant well, so sow where you want it to grow--be sure to include bonemeal in the very well draining potting mix & give nitrogen only if leaves begin to yellow)

Nicotiana (annual--plant the shorter types)

Primula malacoides (These need to be sown in April or May for bloom the following winter.) Try other primroses, too.

Salpiglossis sinuata (Painted Tongue, annual--really beautiful velvety flowers with a wide range of colors, needs staking and pinching--use as a cut flower)

Schizanthus (Butterfly Flower, annual--needs pinching)

Viola (Pansy, annual--try winter types such as Swiss Giant or Imperial Antique Shades)

Try growing any flower or vegetable. It might do well. If it doesn't, call it "an experiment." If it does, call it a success!