Good on you for recommending agapanthus, it's one of my favorites. Also in the lavender/purple scale is freesia, carnations (yes I know carnations, but they come in some great shades now and if you use them as a filler rather than a focal flower you get alot of color on the cheap), iris, delphinium, spray roses, orchids, lupine, dianthus, larkspur, monkshood, etc.
Also, for centerpieces try just petals or heads off of daisies, polished river stones, floating candles, I've also done weddings where the bride had goldfish or beta fish in glass bowls with colored stones. Do your research and make sure you knw what you want before you go to the florist but be flexible. Know that you may set your heart on something only to find out its not available. I know that sweetpeas were mentioned before, as a florist in New England I can't get those anymore in July.
Don't settle, see a few different florists and get price quotes from all of them. Be sure to ask about delivery and other sneaky fees that may get slipped in there. Also, if you have time, go into the shop the day before to see your flowers, some florists get antsy bout this, but it prevents upsets later when you realize that your purple irises are dutch blue.
Hopefully this helped some and congratulations on your engagement =]


per aspera ad astra: Through rough ways to the stars...