There are a lot of flower arrangements, but the 'Scent with' and 'Scent with Rare Flower' options are the most interesting part. They are available for arrangements of Excellent quality (and presumably higher, I haven't seen any yet). The flowers for this must be in the sim's inventory, not stored in the bench.
When giving arrangements as a gift, the giver may also receive Happy +1 for 2 hours (Giving Spirit) and the recipient Happy +1 for 2 hours (Generous Surprise!) as well as any other effects listed for specific arrangements.
Arrangements degrade with time, from Pristine when first made to Normal after 24 hours, and Wilted after 48 hours. Effects from scents seem to stop working when they become Wilted, based on testing the Bee-friending effect only so far.
If you've spotted anything else, feel free to shout up! (Additions from replies marked with *, more details can be found in the thread).
Scent with:
Bluebell Timelessness
Arrangements scented with Bluebells don't degrade, or at least not that has been reported so far.*
No effect when giving as gift to giver or recipient for me, but that may have been because of vampire sim or my age settings.
Chrysanthemum Friendship
Has smiley faces floating away from it. Gives Full of Friendship option, happy+ 2 for 2 hours (Flowers Full of Friendship). My sims also chose to hug when it was around but that could be coincidence.
Given as a gift, the giver gets Confident +1 for 2 hours (The Gift of Flowers) and the recipient gets the Flowers Full of Friendship moodlet above.
Crocus Bee-Friending
Grey mist/smoke effect with bees buzzing around. No menu option. After experimentation, can safely say it keeps bees calm when placed near the hive.
No specific effects seen on giver or recipient when given as a gift.
Dahlia Mourning
Has a black rain cloud over it, and I think thunder sound effect came from it. Menu option Weep From Beauty, gives Sad +3 for 2 hours (Moved by Beauty).
Given as a gift, the recipient gets the Moved by Beauty moodlet above.
Daisy Joy
Has green glowing ray effect. Gives Joy and Sunshine option, happy+ 2 for 2 hours (Flowers of Joy and Sunshine).
Given as a gift, the giver gets Happy +1 for 2 hours (Flower Power) and the recipient gets the Flowers of Joy and Sunshine moodlet above.
Rose Romance
Transparent red hearts effect. Menu option Fill with Passion, flirty +2 for 2 hours (Floral Passion).
Given as a gift, the giver gets Flirty +1 for 2 hours (Love is in the Air) and the recipient gets the Floral Passion moodlet above.
Snowdrop Seasons
Orangey glowing flames floating off it. Menu option Watch Wisps, inspired + 2 for 2 hours (Wisp Watcher).
No specific effects seen on giver or recipient when given as a gift.
Snapdragon Disrespect
Red * and @ symbols fly out of it. Menu options How Dare They, Rude News, Shut Up all seem to give Angry +2 for 2 hours (DISRESPECT).
Given as a gift, the recipient gets the DISRESPECT moodlet above.
Tulip Faithful
Effect is yellow-orange glow with sparkles. No menu option, no effect noticed from being near it.
Given as a gift to a romantic partner, the giver gets +2 happy for 2 hours (Faithful). No effect on recipient. Haven't tested giving it to any sims other than romantic partner.
Scent with rare flower:
Begonia - Cursed
Dark purple spiky glow effect. No menu options.
Given as a gift, the recipient gets Dazed +3 for 24 hours (Cursed Gift!). No effect on giver.
Death Flower - Death
Black mist effect. No menu options.
Given as a gift, the recipient ages up immediately. If an elder, they will die.
Lily Creativity
Has light blue glow with little lightbulbs floating away from it. Gives Creative Skill Inspiration option, inspired + 2 for 2 hours (Inspirational Flowers). Text (error) says it comes from 'Joy and Sunshine Scent Response'.
Given as a gift, the recipient gets the Inspirational Flowers moodlet above.