When visiting Glastonbury, if you seek to have an overview of the island, its surrounding Moors, and all of Glastonbury's holy hills, then visit Wearyall Hill. This is where guided tours around Glastonbury tend to start.
Though this isn't the highest hill in Glastonbury, it affords fine views of Avalon and its surrounds you just have to ignore the supermarket, business park and main road below!
Legend has it that Joseph of Arimathaea (more about him later) landed here, rested on his staff, and it sprouted branches and leaves. This is an old shamanistic tradition indicating 'this is the place to be', where one can set down roots. However, the chances are that Glastonbury was well known at the time, at least to those in the know, and that he had either been invited here or knew it could be a refuge, amongst people who would understand the Christians' plight. Tradition has it that he was a rich metals trader who traded with Britain for tin from Cornwall and lead from the Mendip Hills just north of Glastonbury.
From Wearyall Hill you can see over the town and the Abbey. You can see the other holy hills of Glastonbury too: from the right, the Tor, Chalice Hill and St Edmund's Hill (nowadays called Windmill Hill). You can see the extent of the Isle of Avalon and its surrounding, once-marshy Levels, which made it an island in ancient times.
People could approach Glastonbury from the sea by boat up to later medieval times. The old River Brue (it has now been re-routed along a more direct route to the sea) flowed close in to Wearyall Hill on its south side, then swung round its western end to meander past Bride's Mound, then the Glastonbury Lake Village, then Godney, Panborough Hill, Martinsey, Nyland, Brent Knoll and finally Brean Down a two-day journey by boat to the sea. (More about these in the Sacred Sites section on this site).
Since about 1000 CE sea levels have been sinking, and between the 1600s and the mid-1900s the Levels were incrementally drained to favour farming and peat-digging. However, to the north, west and south of Wearyall Hill, you can see the flat 'moors', which formed the extent of the water and marshes. With 21st Century sea-level rises, the Moors might again become flooded. In winter, when there are prolonged rain downpours, the Levels flood again.
To the north you can clearly see the Mendip Hills, and to the west you can see the marshy woodlands of the Levels. If you get hold of a bicycle, you can ride down an excellent bike-track west of Glastonbury (starting on Benedict Street) into the Levels and what are now an impressive series of wetland nature reserves, with thousands of birds in residence.