I'm in a 482 sq ft condo and also have very limited room to workout - I do pretty well, given the space constraints.
I own one weight - a 70lb kettlebell. I store it on the bottom shelf of an ikea cabinet. Behind this cabinet, hidden from view pretty well, is a thick rubber mat, which is just smaller than the cabinet that hides it.
My nearly-daily workout is simple.
I start by putting the mat over the floor to protect it from being crushed by my kettlebell when I put it down between sets.
As I've gotten fitter, I've progressed from 100 two-handed 70# KB swings in 10 minutes, to 100 two-handed 70# KB swings in 5 minutes, and now to 100 one-handed 70# KB swings in 8 minutes. I plan to keep progressing. I also do Turkish Getups with the kettlebell - a few every workout. For the swings, I use 25 second work intervals and rest intervals that get shortened by me every workout, making it harder every day.
This is nice because with a kettlebell weight heavy enough to challenge me, it's a strength workout. But it's also cardio, particularly when I compress my total workout times to under six minutes. At the five minute mark, I'd end the workout gasping for air, on my back for several minutes.
Other things I do regularly:
1) I have a "handstand wall," where I do handstands holds for time. Two minute holds, three minute rests. Repeated until exhaustion.
2) I have a pull up bar in my bedroom hallway, which I use several times a week.
3) I have a jumprope and walk outside to the condo pool to use it for double unders and singles.
4) I do burpees.
5) I go outside to a nearby park and do sprints.
6) I ride my bike for transport, which passively makes me fitter, especially because I need to ride up a big hill to get back to my condo from downtown.
7) I do partner acrobatics as a fun hobby, which has left me a lot stronger. It's also free.