散步 means 'to take a leisurely walk or stroll' — it's specifically for walking as a relaxing activity, not for getting somewhere. 走路 (zǒu lù) is just 'to walk' as a means of transport; 步行 (bù xíng) is the formal, written equivalent of 'to go on foot'. 溜达 (liū da) is a more casual, colloquial synonym for strolling.