Wife had an old 3GS which ended up in her lunch bag along with a half full coffee travel mug. Mug leaked, phone was soaked. Bought the wife a Samsung 4G (which I now think she planned). Decided to fix the 3GS and give it to my 5 year old minus the SIM card.
Replaced the battery, home button switch which had been intermittent, and waiting on a new speaker assembly. Home button was a pain as the switch I received from another vendor was missing the tiny raised nub which contacts the button cap. Had to make this part out of epoxy. Worked out fine. Unfortunately ifixit did not stock the 3GS home button switch so I had to look elsewhere. The battery pull tab needed a little trim with an exacto knife as it was a bit too big. The main board would not sit properly. No big deal, modification took 30 seconds. Got the phone working only to discover coffee took its toll on the hands free speaker. The tools I ordered made things so much easier.
Take it slow and don't drop your screws on the carpet. Spent 10 minutes with a large magnet trying to recover 3.
iPhone 3G and 3GS Front Panel Adhesive Strips
iPhone 3GS Replacement Battery
Mako Driver Kit - 64 Precision Bits