My husband & I will be visiting the gardens in early May-any advice for seaside resorts, length of garden time (days-me; hours-he), transportation, buy tickets from home (US), bike/walk to and from??
You can purchase garden and combo transport tickets at the official website, and print your ticket out at home:
The gardens aren't in an urban setting, they're surrounded by lots other farms.... not much else to walk/bike to.
Get to Keukenhof early in the day. It gets quite crowded by early afternoon. The indoor displays can become packed. If you arrive before the mid-day crowds, depending on how long you like to linger, you can see it all in about 3-4 hours.
You can buy your tickets from home, but there is no real need. It doesn't sell out, and even if you arrive mid-day, the ticket queue isn't particularly long.
The most popular seaside resorts in the Netherlands (and the only ones I know of, although there are certainly others) are Scheveningen by Den Haag and Texel, an island north of Amsterdam.
Re-reading your post... as noted above, Keukenhof sits in the middle of the polderland and the surrounding towns aren't particularly interesting. However, this being the Netherlands, you can bike just about anywhere. If you are in decent shape, Leiden sits only 14 km to the south and Haarlem 17 km to the north. Either city makes a decent home base for visitors.