Hi, for solo trip do go for full moon night, beach party at Koh pangan or half moon jungle party. Take the traditional boat rides to phi phi island. you can even plan to stay there for 2 nights. Also, you can take up 4 island or 7 island boat trip from Krabi. Otherwise there are lots of boat ride option available from phuket, you can check at any store selling this trips. In phuket stay near bangla street and the party scenes are awesome out there. Krabi is more like couple's place, parties scenes are sober out there.
Pattaya is definitely better than Phuket in my experience. Phuket has a very laid back vibe and unfriendly people. its also far from Bangkok. pattaya is close by, lots of water activities but tired only underwater sea walk not scuba diving. so no idea about scuba. party scene in pattaya or Bangko
Hi, Phi Phi island is awesome for beach party.. whole shore will have DJ, fire shows, Ibiza arranges pool party(check schedule). Don't take tour for phi phi, but take a ferry from phuket and stay there. Keep half a day for scuba if you plan. There are vendors at Phi Phi for Scuba.