The science centre is at Don Mills & Eglington. Just take the Eglington Ave exit from the DVP (Don Valley Parkway) and head west one block. Can't miss it, south west corner.
Anyway, for cool stuff to do it really depends on your interests. The World's biggest bookstore is right downtown around the corner from the main HMV store (were the bassment is), the waterfront has some nice areas, etc. Toronto is really an adult city so if you're not 19 I'm not sure what to recommend.
If you have the cash having dinner in the restaurant on top of the tower is still a pretty cool thing to do. The food is actually good now too. You looking at about $80 a person as a price range with no booze though, so it hurts the wallet a bit.
Anyway, Yonge street around Dundas has enough to do to keep you busy for days. Music stores abound, the book store mentioned earlier, the Eaton Centre, endless sports shops, elctronics shops, Ryerson University. Tons of stuff. If you into funky stuff check out Queen street around Bathurst.
/advice
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