Bush restoration: planting, weeding, running a small nursery, running a small charity nursery for erosion control separately. Takes up a lot of my time (a nursery is basically looking after babies).

Relatively low cost, even the nursery after some minor expenses in the bulk potting mix and fertilser. Technically, if you costed up the finished product, $100 of bulk potting mix in back of ute can make thousands of dollars of plants so the cost is a positive if you value it like that. I found all the pots/tubes by asking anyone I knew for their old ones.

And then when I plant them or give them away I feel like I’ve contributed in some way to the problems we all face. Regenerative rather than sustainable rather than degenerative. I think a lot of people should look at their lifestyles and see how many things they can put into the regenerative column and try to increase that. For example, veganism is sustainable not regenerative on an individual level. We’ve been waiting on governments (all levels of) to step up for decades and it hasn’t happened yet plus if you look at !environment not much of the news is at all good (and most of it that is posted is completely recent, I haven’t gone back in time much for older or shittier news). Private landholders aren’t really doing the job either.

Join a Bushcare or Catchment Group that take volunteers and give it a go. Sure, you might be working on some old boomer’s property but you’re benefiting the environment, not them. Be like the Lorax and speak for the things that have no voice, the plants and animals.

I love to do a big cookup. Last night I cooked up some butter chicken and lamb korma. Gave us last nights dinner plus 7 meals in the freezer. If you gotta cook anyway why not make a bit extra and freeze it?

I love my treadmill. It’s a huge upfront cost, but cheap-as after that. Now, watching TV/Movies is not something to feel guilty over.

Watching a movie while walking will see you do 12,000 - 15,000 steps - twice an average adult step count. It’s great for your body. I don’t even notice that I’m exercising.

Most of my hobbies are ones that can be very expensive, but don’t have to be. I’ll start with a couple that I do this time of year even though they’re outdoors.

I’m big into running. Good running shoes can run you a lot of money, but you can get a pretty cheap entry-level pair and keep healthy without any ongoing cost. Save yourself that expensive gym membership. It’s hard to get out the door in winter, but once you do it’s a great experience because you don’t end up quite as drenched in sweat as you do in summer. As @Ilandar mentioned, Parkrun is available as a free race every week if you want to have a benchmark to test yourself against, or want a good social opportunity. It’s 7am in Qld though, not 8am. I know NSW is 8am, but outside of those two states I don’t know, so check your local races’ websites.

Photography. If you’re really into it you’ll probably eventually want a DSLR or interchangeable-lens mirrorless, plus some lenses, but you can do a lot of stuff with just your phone, if you already have a decent phone. Sports and wildlife photography will suffer the most without better gear, but landscapes, cityscapes, candids of people and a bunch of other styles rely much less on the technical capabilities of higher end gear, and proportionally more on just composition, which a phone can do equally well. (You just gotta git good.) You can rug up in your warmest clothes because there’s no need to be moving around very quickly most of the time (again, unless you do sports photography). Or you can explore creative ways of taking photos of things in your own house for a more deliberate artistic expression.

Since you were mainly asking about indoor activities, the obvious classic answer is video gaming. Yeah you can buy half a dozen new games every time there’s a Steam sale and run them on the latest and greatest hardware. But you don’t have to. I’ve pretty much only been playing 2 games over the last 4 years: Age of Empires 2 and Age of Empires 4. They run great on my computer which was a moderately powerful gaming PC when I built it in 2017. AoE is a great game because you can have a blast just messing and doing campaigns or beating the AI, or you can take it very hardcore serious on the 1v1 ranked ladder, or you can enjoy it with friends via the ranked team game ladder or by doing teams against the AI. I do all of these depending on my mood.

Parkrun. It’s a free, timed 5km run held every Saturday morning at 8 AM. There are courses all over the world and once you can attend any of them for free, without needing to register again. If you are an extrovert there are lot of people to meet and you can get into the volunteering community too (125 runners at my local on the weekend, despite cold and wet weather) but if you’re introverted like me it’s just fun to have a weekly run with/against other people. Because it’s timed, you can also track your progress through your online record and there are running and volunteering milestones to aim for too. If you live near a major city like I do, there are also tons of local courses to “tour” when you feel like something different or want to meet new people.

EDIT: I realise now that you specifically wanted warm, indoor activities. Maybe revisit this one in Spring lol