Well I was reading your thread wondering what my DS will likely be into when he gets a bit older. But it seems like probably mostly the same stuff as right now judging by what you are all talking about.
I've found toy libraries and toy hire places to be great when you want to change up their toys without having to spend a fortune and also not having to store everything once they get bored of it. I'm not sure if they have that kind of thing set up in your area?
We typically get a new toy once a month and take back the old one when we get the new. The prices are pretty reasonable and a fraction of what you'd have to pay for it from the shops or even second hand.
My DS loves to explore everything which includes taking things apart and examining all sort of things. He just loves mechanical things and has a set of cogs that stick to the fridge which he loves, will spend ages standing there arranging them and pushing them together and watching them turn and turn each other. He is also just sooooo interested in the inside of toys - their motors and stuff. DH gets out the screw driver and takes something apart and he'll be engaged for half an hour or more. Even just taking out batteries and putting them back in.
I think dont be too afraid to give them things that you think might be "too old" just because it says 3+ on the box. (All the good toys say that LOL) If your DS is bored of Duplo why dont you try for the older age lego? We give our 18mo the normal lego for 5-12! He loves it. Perhaps he needs to stretch his skills a bit more and needs a new challenge.
If your DS likes farm tracktors and those sorts of mechanical things then try to find something that has good moving parts that are a bit older. I know the brand Catepillar have some decent toys - replicas of the real thing, although thats mostly diggers and dumptrucks but maybe you can find a John Deere equivalent?
My DS is obsessed with the iPad and also loves watching youtube on it. Luckily for us (so far!) he associates his entertainment watching only with the iPad and not so much on the TV.
Oh here's another thought - dare I say it - what about a Nintendo Wii? They've come down in price a lot lately, and they're actually not a bad workout for indoors! If your DS loves the TV then the Wii is at least a more "active" option than just sitting and watching. You might be creating another monster, but it would probably buy you a bit of time for the new baby!!!

The best part about the Wii is that you really dont need alot of games, the sports one (and also sports resort) that usually comes with it have lots of different sports games in it that I'd bet wouldnt take much for a 2.5yo to learn. Generally you just need to wildly shake the controller. hehe
Dont get me wrong though, normally I wouldnt advocate computer / consol games for a 2.5yo but I realise you've only got a short amount of time to find a few good options before #2 gets here! HTH