What is the best way to treat a rest day?

As I get older I really need to make sure I have a rest day after a hard run or race.  Sometimes it is two days after a really long or tiring race. But the problem is what to do on that rest day - nothing? I did the club handicap race last night where faster runners set of at suitable intervals after slower runners. I ran hard and even though the undergrowth was dense at times was only 8 seconds outside my PB on that course.  So understandably my legs are a little sore this morning and I need a rest before doing a 5 mile trail race tomorrow. So what to do.  If I do nothing I will feel lazy and I actually find that I stiffen up.  I don't want to run as I feel that my joints, tendons, muscles etc will complain.  So the plan is to go for a longish 3 mile walk off road and then do some Yoga later on using my favourite Yoga App - Yoga Studio