I'm basically with Kenat on this one. I actaully enjoy writing Excel spreadsheets, so that is a good one for me. Many times I will write one for the tedious, but simple calculations. Also, I like to solve problems I've come across in the past that I didn't get to completely kill. Any issue that was resolved based on a rule of thumb, senior engineer's advice, or doing it the same as has been done in the past. I will go back and run the numbers the best I can and see how things match up. Sometimes it turns out that when I start to try to solve it analytically, the problem is very complex and a rule of thumb is just as accurate as the assumptions I would need to make. Sometimes though, it turns out that we have been designing with a factor of safety of 82 or something absurd because nobody bothered to try to figure it out and just made it stout.
Also, I will often make lists of things I could have done better in the past to look back on when there is more work to do. For example, don't just send the customer the utility requirements for a piece of equipment we are designing. Send the drawings/req.s, get a signature, and actually talk to the person who will be responsible for ensuring it gets done.
-- MechEng2005