A site for red kites that is not too far from Aberdyfi is Bwlch Nant-yr-Arian Forestry Visitor Centre at SN718813.
As a regular visitor to Aberdyfi I can't recommend the above site too highly - you'll get great views of the kites here.
Closer to Aberdfi itself, try Aber Dyssinni near Tywyn, 4 miles to the north. The rivermouth is good for Terns (usually Sandwich) and waders (try scoping the rocky beach on the north shore) - Early is best as there can be disturbance from dog walkers and fishermen. The lake inland (Broad Water)can hold a good variety of widfowl and waders depending on the amount of disturbance. Kite, Peregrine and Raven are regular here too. The sea should have Eider and possible Divers whils offshore there will be Manx Shearwater and Gannet depending on the weather. Skuas are sometimes seen but usually later in the year.
Inland, Craig-y-Deryn (sorry my Welsh spelling is awful) or Bird Rock as it is called holds Chough, Peregrine and Cormorant.
Red-breasted Merganser are common on the Dyfi estuary, as are the commoner waders. On the far side of the Dyfi (but you'd need to drive round) Ynys Las is good for seabirds and the inland Borth Bog area can hold Water Pipit, Raptors and passage birds such as Whimbrel and Whinchat.
Dolgoch Falls, inland from Tywyn is typical Welsh oak woodland with Redstart, Pied Flycatcher, Wood Warbler and Tree Pipit as well as Dipper, Grey Wagtail and the usual woodland birds, you might be too early for some of the migrants but any of the woodlands round about can be as good.
Aberdyfi is a beatiful spot in its own right - the birds will just enhance it! Watch for the gulls and Jackdaws going up in a panic - it can mean Buzzard, Kite or if you are lucky Osprey flying overhead.
(If you have the time and money try the food at the Penhelig Arms - it isn't cheap but the bar snacks emulate the restaurant and can be fantastic! You can sit outside and watch the birds on the estuary from the wall)
Feel free to PM me if you need any other help.