A few recommendations:-
Beaumaris – Castle, Gaol, Crabbing on the pier, Ice cream from Red Boat.
Menai – Food and a walk under the suspension bridge
Newborough Beach / Forest & Llandwyn Island.
Holyhead Nature Reserve and Beach
RAF Valley check website for watching the fast jet training and viewing points.
Holyhead South Stack – RSPB cafe closed at mo.
Around Anglesey walk. Walk from South Stack to Treaddur Bay and onto Rhoscolyn is excellent.
Wavecrest Cafe at Rhyd Wyn – Church Bay
Parys Mountain @ Amlwch
As others have said no much in H/Head but u can walk the breakwater.
Anglesey Transport Museum
Enjoy your trip.