> How do I go joyriding?

The only time I can think of where you need to drive something is when you drive your car up to the GSIP place and the store, and you've already done that, so there's no reason to worry about it, unless I'm forgetting something else.

> Where are the rusty key and sledgehammer?

Check your inventory -- you already have the rusty key. As for the sledgehammer, that's one of the professor items -- since you didn't select the professor who gives the sledgehammer, you don't need to worry.

> What do o need to trade and where do I get them?

The only thing I can recall that you can trade in this game is tokens, and that's one of the professor challenges -- since you didn't select the professor who gives the tokens, you don't need to worry, unless I'm forgetting something else.

> Do I need to go to the health centre and look in the trench again?

I don't think so.

Lucky "What you have to do is find an unexpected route to that bus" Wizard