"This park is more beautiful in spring."
Translation:Ce parc est plus joli au printemps.
This applies when belle/beau is an adjective before a noun that begins with a vowel too. http://french.about.com/cs/grammar/a/specialadjforms.htm
Dans means in specifically in the sense of in a place or how long until something will happen. For time, you need either en or durant. This is pretty good run-down on the subject: http://french.about.com/library/weekly/aa040100.htm