If he's not got much Duplo, I would say a couple more sets of this for sure. We just took a box on holiday and it kept O amused for HOURS. He loves sorting the colours, counting the bricks, and has really just started to use his imagination with what he builds. We have a couple of 'Bob' sets which are great - Lofty, Scoop and Muck aal in Duplo with Bob ey al.
O is also loving making much more elaborate sets with his wooden train track, so that's a definite goer as well.
As far as days out go, my MIL lives in France (thank feck 
) and she always sends the boys a cheque - O's will be spent on his zoo pass for the year so he has something that lasts a long time. Do you have somewhere local that you could get a pass to?
O also has a My First Leapad which he really loves to use now - again, he's grown into it and it's great for hand-eye co-ordination and thought processing
K'nex has also gone down well (cheap sets always in TKMaxx)
I'm struggling for ideas for E's 2nd birthday at the mo, so will keep an eye on any further ideas!