J.O.
Routine, routine, routine, it isn't the same as a schedule, that is much more rigid, but some consitancy through out the day will help, the routine will have to be up to, look at an average day, what time the baby usually naps, what time they are hungry ect and start following there cues, when the baby is napping spend some time with the toddler, read some books, color anything he likes to do, then have him sit and enetrtain himself, also important, while you do a few things you need to do.
I think you are handling the store perfectly, since you haven't been doing much dicsipline it is going to be a harder road, but not impossible, if you really want to get things under control then you are going to have to say to yourself "enough" and stick to it, rememeber you have a baby who is watching and learning everything big brother is doing, if you don't want to have to little "hellions" on your hands you need to be consistant. time-outs only work if done right,
1.Place child in seat and say to him that "insert, is not OK!"
2. If child gets up replace in time-out, the first 3-4 days are going to be hard, time doesn't start until he sits and doesn't get up, you will keep having to place him, sometimes 20 times until he gets it, it will get better as he learns that you are not going to give in.
3. Once his time has been completed, get down to his level look him in the eye and repeat why he was there, ask him if he understand why, then give him a big hug and kiss and let him know you love.
It is hard at first, but if you don't give in it will work.
Also to the people looking at you in the store...WHO CARES! The only person who looks like an idiot are the parents who let there kids run wild and have no control whatsoever, I am not talking about kids who are playing and bugging no one but we all know who I am talking about. Good luck!