I grew up on some fantastic old school SF (Heinlen's "The Rolling Stones" for example), but as for modern stuff no-one has mentioned so far I would heartily recommend the Artemis Fowl series by Eoin Colfer. Artemis is a 12 year-old criminal mastermind with a ninja-bodyguard butler, and he discovers that the earth is hollow and strange beings live in there.
Tamora Pierce writes some good modern fantasy, like her Alana series...mmm, it's actually called "The Song of the Lioness Quartet", four books (and then a couple of extras) with a young female protagonist.
My four kids (one boy, three girls) all loved these, along with the usual Terry Pratchet's/Harry Potter.
The multiple people recommending the Tom Swift series also reminded me of a very old series I loved, the sort of thing you graduate to after Encyclopaedia Brown: Rick Brant Science Adventure stories. Probably hard to find, and definitely 'long in the tooth', but I adored them at his age.
Those old Retief books were fun SF books when I was a kid, and are still funny to re-read as an adult (and get all the sarcastic digs at bureaucracy and diplomats that you missed as a kid).