So we did our Keukenhof visit today.
We didn't get started until late. The wait for the bus at the airport was 45 min or so, (we got there about...noon-ish) and unfortunately, we ended up having to stand on the bus, and the driver was sadistic with the heat cranked. Ugh. Not fun. We should have waited for the next bus at the head of the line so we could get a seat.
So, we decided to do the bike tour first. The parking lot was pretty full and a lot of people going in. We arrived back a little before 4pm (we got slightly misplaced on our journey...you really gotta watch out for the direction signs!). Absolutely beautiful.
The displays were not at all crowded, and lovely outside. From what I've heard others say, I think we made the right choice to wait until later. We overheard a couple the day before telling someone they arrived first thing, and by 10am, the crowds were nuts.
We stayed until about 6:30 or so. I was getting all flowered out by then, but so worth it. And my butt cheeks aren't happy with over two hours on a bike...lol.
So I'd recommend doing the bike tour first if you don't get there until mid morning (if the weather is good) and maybe going in mid afternoon.