Flowers should still be great this weekend (Happy Easter!). When we were there at the beginning of the week it was very nice, but there were definitely some things that had not opened yet. The Opuntia beaver tail should be quite spectacular. Also on your way into the valley there will probably be Joshua trees in bloom, worth a look.
Without a reservation you will be quite limited in where you can camp. What will be available will be basically gravel parking lot. If you are adventurous, there are a number of boondocking places you can camp, but you must go quite a distance and over gravel roads to get to them, no water, no tables, no toilet, no nothing but a beautiful place.