Willard Price: Hal and Roger Fillybuster dd2 loved them.
Alex Rider and The Cherub Series definitely
Possibly Charlie Moon (Jenny Nimmo), Dd1 hasn't enjoyed them, found them a litle bit disappointing after the first.
Malcolm Saville books are great adventures. He also wrote other series including the Marston Baines series, which are spy books. Buckingham series are a little girly, I think, but the Jillies books he also might get on with. Ebay usually has Armada ones at okay prices, they're okay if he wants to try, but the hardback/GGBP are unabridged and much better adventures imo.
A little old fashioned now, but still great-I've just lent the first 5 to a friend in his 60s who hasn't read them since he was a child 
Hardy boys
3 investigator
Joan Aiken's Bridle the Wind/Saddle the sea/whatever the third one is trillogy.
Young Bond
Other Enid Blyton-the adventure series is a bit harder than the Famous Five, or there's also the Five Find outers.
Arthur Catherall wrote some good adventure stories-again out of print, but usually some on ebay.
Ditto John Putney (I think) the Fred and I series are great adventures.
For the Swallows and Amazons, I could just about get the 9 year old sleeping on the island, but what about "Secret Water" where they're marooned on the tidal islands with no grown ups at all including Bridget who I don't think's older than about 5yo. 