I don't know if this would be a possibility, but could you leave the bigger kids with Grandma and take the little one with you? 2 year olds still fly free (I think) if they lapsit. When my oldest was about 25 months, my husband planned a weekend to San Franciso for us and I just told him I wasn't ready to leave the baby for that long.
You'll only have one little person with you if you take her which is much easier than taking all 3.
If you can't take her, what the others have said was good. Make plans for special treats she likes, like going to the park or going to a favorite place to eat while you are gone.
The other thing to keep in mind is, it's harder on YOU than on them. When we went to Hawaii for a week about 4 years ago, my boys were 2 and 5 and although I really wanted to go on the trip (I'd never been to Hawaii) I was worried about how they'd do for a week with Grandma and Grandpa in a different house and city than they were used too. Let me tell you, every time I called to see how they were doing it was like pulling teeth to get them to the phone...they were having WAY too much fun and didn't want to talk to me. LOL Of course they were thrilled to see us when we got back (they came to pick us up at the airport with G&G) and they told us about all the fun things they had done while we were gone.