Yes, everything can be mediatative but it's more the matter of how you do it. You can do gardening but still renumerate about past and future. It's all about focusing on what you're doing and nothing else, centralising all 6 senses to one point of fucus.
"a walk with just your thoughts" can lead you to many places include good and bad places. A mediative walk is to focus on nothing else but your body including your breath, your arms, your steps, the sensation of skins, the smell you breath in and out, the sound you hear. You don't need to focus on all of these at once but you can just pick one and focus on it for a period ideally as long as possible, but you can switch between them at the start. All of these is very difficult but not impossible if you do in a crowded city when everything changes very fast and you're exposed to many things that you tend to like or dislike. Of course, if you're a zen master, you can meditate everywhere but personally I feel I'm very far from it. The fact that I struggle to focus even in quiet places tells a lot about how much I need to practice.
this is similar to Kasina meditation, one of the oldest concentration practices in the Buddhism. You focus on one of 10 subjects including Earth, Water, Air, Fire (elements), Blue, Red, Yellow, White (colours) and Space, Lights. These subjects can be found anywhere and most importantly neutral so you won't or unlikely to have sensual attachments and association. This is considered an entry to mediation. It's easier than focusing breathing because breathing is movement and very natural so you're likely to lose focus. The biggest advantage of breathing to 10 entry subjects of Kasina meditation is it's everywhere. Effort or single-mindedness or focus is one of 8 noble paths in Buddhism. And meditation is to improve focus/concentration. At least that one of the purposes of meditation.