Tickle it!
Sure, I sometimes have stuff show up and I'm not sure if its actionable yet, so I'm not sure what I'd even put on a Someday/Maybe list, but at the same time I know that if I slide it into my reference filing system I may not see it for a year. For these items I use a tickler file. I'll even forward things one day at a time until I make the decision on whether I'll use it. You can cycle something through a tickler file indefinitely until you decide to do something with it or throw it away but it doesn't take up any psychic RAM whatsoever.
Here are a few options for the things you mentioned. I'll assume that you're using a paper system.
GTD mastery 100 checklist
Put it in a file called "Checklists" and review as needed (at least during the weekly review) or take it out of your inbox, review it as a 2-minute next action, and place in tomorrow's folder in your tickler file. After you're done processing your inbasket at the end of the day you'll dump tomorrow's tickler file into your inbasket and the checklist will show up the next day when you process again.
Something else im waiting to hear back from
If the paper article is nothing more than the reminder (you or anyone else doesn't need to see it before or use it after you hear back from someone), you can drop it in your tickler file on the date that you would want to trigger a next action to follow up on it. Otherwise put a NA on your @Waiting For list or put a note on a separate sheet of paper in your tickler file and place the original article in your general reference system.
A 20% off when you order online offer
There are a few ways to handle this. I'll assume that you have absolutely no idea when you will place an order but you don't want to forget that you have it. You could print off the coupon and put the printout in your tickler file and keep forwarding it until you use it or it expires and you throw it away. You could also file it either electronically or on paper and put "Redeem coupon for x" on your Someday/Maybe list. That way you'll at least review it weekly.
A timetable for gym classes which i would like to keep at hand - anywhere where i know where it is is fine
I'd put this in a general reference file and carry it with me in a travel file pouch or "accordion".
"It's OK to decide not to decide if you have a 'decide not to decide' system" - David Allen
I hope that helps.
