Any ideas

I’ve got a 2 year old boy who very smart he loves doing the toddler puzzles but any other idea on what to get him that would challenge him or some even fun that u may have got ur kids for charismas Also got a 5 month old baby he’ll be 6 months this month but he does not sit up only holds things for a few seconds I do believe he may be late on things he’s also only a 13 pound about 4 oz baby any idea other then tether to get him I’d love to get him toys for 6-12 month babies that he can play with but giving the back he doesn’t sit up only his own he literally just learns right on over didn’t hold anything for more then maybe a min I’m not sure what to get him any ideas ?