For plants in flower all summer long, you need summer bedding. These are tender plants which can't be put out before mid to end of May, and will die in the frosts in autumn/winter. Seems a waste to use large pots for those, but your choices are petunias, surfinia petunias, bush or trailing fuchsias, pelargoniums, regal pelargoniums, ivy leaf geranium, cascade geranium, bedding begonias, corm type double begonias, lobelia, alyssum, verbena, pansies, New Guinea impatiens, French marigolds, African marigolds and other plants - available as box bedding usually from about now onwards. Dahlias also, though these are available usually as single plants or rhizomes. Note that the vast majority of these are not bee friendly, being sterile (without pollen), with the exception of open, daisy flowered type dahlias.

80cms is a big pot, Tim! I couldn't possibly improve on Bamboo's list, however, you might like to think about giving the group you plant some height, possibly towards the centre, with something like a cordyline, which, of course wont give you any flowers, but sets off bedding plants beautifully, or, as Bamboo suggested, dahlias can give a good height, although some of these won't really come into their own until towards the end of summer. Something I used last year with my bedding plants in pots and tubs were trailing carnations: the flowers went on and on, right through summer, and were still in bloom in November when I was clearing the pots out, and the perfume was incredible. Unlike most bedding annuals, you can take cuttings from these very easily and overwinter them, so you'll have more for the following year.
You don't say where you normally site these pots. Are they in a sunny place or will they be in shade for some of the day? It makes a difference to what will do best.

80cm diameter is a lot - but, Gattina, we don't know how tall they are - they might be very wide but shallow...

Very true, Bamboo - I was thinking of similarly wide ones that we have. They take a lot of filling!