For students, what is the most accessible archaeological site (and/or museum) for a brief (two-day) study of Roman Londinium?
Without a doubt, the Museum of London.
For starters, it still has part of the old Roman walls standing outside the museum, unmoved over two millennia.
It also houses an impressive Roman gallery.
In addition, they have an exhibition "Our Londinium" running:
And...it's also free :)
Near the Museum of London is the City of London Guildhall Art Gallery... which happens to have a genuine Roman amphitheatre in its basement.
Requires some imagination - it's not exactly intact - but it's the real deal. Very occasionally it's used for events including theatre productions, but that's very rare.
Be aware that with building work in the area, a lot of archeology is being done on old Roman sites, and a lot of interesting stuff is being found, including whole streets - though naturally this isn't usually open to the public. Much of the archiving and analysis is done at the Museum of London, who may be able to answer questions on it.
There's also a Roman Britain section at the (free) British Museum.