What to send
A camellia beside a rose is the traditional thank-you pairing — appreciation with warmth. If you want something less romantic, a peony with a daisy does the same job in a lighter register.
For a colleague or a manager, keep it to one or two stems. A large gesture in a work context can read as a transaction rather than thanks.
Say the specific thing
Three sentences is plenty: what they did, what it cost them, and what it made possible. “You covered my shift on Friday, which meant I could be there when my sister landed. I know you had plans.” That is a thank-you that will be remembered.
Avoid “I really appreciate it” as the whole message. It is the closing line, not the message.
Good moments for it
After someone hosted you. After a favour that went further than expected. After a reference that worked. After a year of small things nobody else noticed — the last one is the most under-used thank-you there is.