Hi BB
As you probably know its a tidal island owned by the National Trust. You can walk on to it along a causeway at low tide but need to phone the NT first to get permission (01621 853142).
The scenery is good but there is in my view no 'must see' sites on the island (no Gothic Abbeys/ haunted castles...)
Personally I think the views are just as good all along the main river sea wall.
I would suggest parking in the Promenade Car Park at Maldon. From there walk along the promenade itself on the river side of the park (great statue at the end!) and great spot for touristy photos. Here you can have great views of wader flocks (Golden Plovers) as well as really close views of duck if the tide is on the way in. There has recently been a Peregrine bombing around.
Then I would suggest walking East along the sea wall- past the yacht club and then the tip and then you may well have the sea wall to yourself for miles. There can be fantastic close views of Brent Geese and also Wigeon, Teal, Little Egrets, all assuming the tide is reasonably high. If tide is low, the Geese may be on adjacent fields. You may end up with views of Geese flying low over head between fields & the river. After half a mile or so you get to the Northey Island causeway. If you can carry on the sea wall for another half mile or so you pass Limbourne Creek. Just beyond there have recently been 2 very photogenic Short-eared Owls as well as a difficult to see Hen Harrier.
Good luck!
John