OK I am biased as I live in Lagos, but I do think that Lagos fits yor bill.
We has a steady throughput of young travellers from the US, OZ, NZ and Canada, most staying at one of the various hostels here in Lagos.
We have excellent beaches, including the 5km long Meia Praia Beach.
At night there are several bars and great restaurants where everyone hangs out.
Lagos is less busy than Albufeira and more attuned to an varied International crowd.
List of Hostels in Lagos here: http://bit.ly/ci0kyk
One downer though is that getting to Malaga from here means either hiring a car or changing buses in Seville.
This link may be helpful: http://bit.ly/cYBCK1
Also Malaga itself is a port city. Nice as it is, I'm not sure you shouldn't be looking at places down the coast from there like Fuengirola, Marbella etc.
