Seal all of the electrical cover plates on your exterior walls. For less than $5 you can buy foam insulating pads that fit under the cover plates and seal off any drafts coming through the wall cavities.
Check your fireplace damper and doors. The fireplace should be completely sealed off when not in use or it can steal a lot of heat from your home. This draft stopper costs about $50. Be careful and wait for the fireplace to cool off before you install it, but you will be amazed at how much warmer you room will be with the fireplace sealed off. Of course you can use insulation or some other material to seal it off but you may find it is not easy to remove when you are ready to use the fireplace again. A link to buy on-line
Caulk around windows and trim. A tube of caulk is less than $3 and with a good caulk gun and 1 or 2 tubes you should be able to get all your windows caulked. Check for leaks around the window trim, too, not just around the windows.
Stop drafts under door with a door draft stopper. You can buy one for about $15 and if you’re at all crafty make yourself a cute one like the little dog in the picture. You can put these on window sills, too. A link to buy on-line here
Insulate and seal attic access panels. You can glue a piece of solid insulation foam board to the back and seal around the edges with removable sealant. Just peel it away when you need to access the attic. A link to buy on-line