I'd consider the minimum free-range time to ideally be at least 90 minutes, although it varies depending on the rat. All my current rats can stay awake and active for hours (makes sense considering that they mostly just sleep in the cage!), so I try to have them out as much as possible. I'm only allowed to take out the rats when everyone else is done using the basement )where they live/free-range), so I usually take them out between 10pm and 1am. On weekends they sometimes stay out longer, but it really depends on what I'm doing. I'd also say that I only really interact with the one on one for 20 or so minutes (training time!), and the rest of the time they usually play their own games, occasionally coming over for some treats/play/cuddles.
As for the cage, while its true that you want to stuff it as full as possible, I find that my rats mostly just sleep in it. I read somewhere that rats sleep for 13+ hours a day, and while I haven't exactly measured this for myself, my girls certainly sleep a lot. My rats will get up sporadically throughout the day to eat, drink, or use the restroom, but that's about it. Occasionally they'll wrestle or play, but mostly they just groom and sleep. Then at around 9:30pm they all get up, finish their business, then wait impatiently by the cage door for me to let them out. And once I let them out, they'll run around for maybe 30 minutes before stopping by to beg for treats or to do some training. After that, they go on playing for some times, upon which my older girl Blackberry usually asks to go back to the cage (when she jumps onto my shoulder, it means she's ready to be put away). Sometimes the others will ask to go back as well, but usually my other 3 girls play until I have to put them away. And that's basically it!
My 4 girls live in a single critter nation, and what I do is stuff it as much as possible. I'm talking multiple dollar tree baskets acting as levels, bathroom wire shelves zip tied onto the doors to create mini levels, DIY basket houses, hammocks, space pods, boxes, ladders, and whatever else I can fit. I used to include a wheel, but my girls don't use the wheel very much and it was just taking up valuable space. So I removed it and replaced it with other hides like igloos or bird tents (my rats LOVE sleeping in hanging bird tents, but destroy them in just 2-3 days!)
My current cage setup looks like this:
I know it looks pretty stuffed, but in my experience rats would rather have a more cluttered cage rather than a bare one. They always find a way to get where they want, although as of recently I've been including more ramps to help my older girl Latte, who has weak hind legs (if Latte didn't have these issues, I would forgo the ramp and instead put more lava ledges/wooden ledges).
As for what toys you should get, I say go for the space pod! When I got my first space pod, I wasn't sure if my rats would like it. I had read all the glowing reviews on the product, but as you know rats can be picky about what they like. But they LOVED it! I've never seen such a universally rat-loved item, heck, even when my girl Cream was so skittish that she refused to sleep in open hammocks, she loved to stuff the space pod with nesting material and sleep in it with the other rats.
Keep in mind that rats aren't big fans of chew toys in general and will usually ignore them. The only exception I've found is the eco cheese chews, which are cheese flavored, edible, and just about the tastiest thing ever in my girls opinion. They go crazy for those chews, and I like to give them 2 or 3 every once in a while.
Another thing rats love are bird toys. Anything that lets them climb (like ladders) or anything that gives them an extra platform (like bird swings/perches/hanging huts) are bound to be favorites. Chinchilla ledges (lava ledges) and wooden ledges are also favorites among my rats. In fact, my rat Blackberry likes her wooden ledge in the cage corner so much that when its warm enough, she'll sleep there 90% of the time!
As for treat balls, those aren't that big of a hit with my girls. They tend to ignore them inside the cage, and only sometimes interact with them outside of the cage. I once saw this cute little contraption in someones rat cage where you could load it up with treats, and whenever the rat pushed a lever one treat would fall out. I think that would be a big hit with my girls, considering they already know how to give paw for treats. Your rats might like the treat ball though, so give it a go if you want. You can also try and make your own "treat ball" by putting treats in an empty toilet paper tube, poking holes in it, and letting them figure out how to get inside. My girls never really took a shine to this either, but then again one of their favorite tricks is finding treats hidden between layers of paper towel, so its not like they don't like foraging.