I have the same issue sometimes! I would recommend the app Dragon dictation, you can voice record your thoughts and then send them to yourself by email or text message.
Either way I am very fond of writing and regardless the trouble of putting down the groceries bag I like to assume a certain kind of tranquility at the end of the day (when I'm walking home) so I take the thoughts' writing as a momentum to enjoy, if I really consider that writing down that thought is really what I should be doing!
Instead of juggling between groceries bags, a cell phone/notepad and pen/voice recorder...I just assume that "Oh this is really a good idea! I want to write it down! It take me about 20 sec. to put down the bags and another 20 to pick them up! In between I take my notes, attentively and dedicated, and may also look around and enjoy the moment. Then I resume my walking home! If your walking home after a work day why hurry?
PS: If you have an answering machine you can always call yourself and leave you a message with a bunch of great ideas (along the day too) and you'll have a pleasant surprise when you arrive home, sit on the couch and listen to your own inspired thoughts