No Problem
You will not having any problem to enter Canada, I hope you need Canada Visitor Visa to Visit Canada,If you enter canada by border no need to submit your Valid I-94 at border while entering Canada if you are going for less than 30 days, If you go by Air you have not to surrender your I-94 to airlines staff just inform them that you are visiting canada for few days. When you ener US your Vilid visa by ex-employer and valid I-94 will be enough but keep your current H1b extension approval with you and a proof that you are working with the new employer in US it can be few recent pay stubs or job latter from your current employer stating that you are working with them.no need to get your passport stamped with new visa.
Good Luck!!