Death-by-squat. Use any added weigh you could find AT HOME.
Vests, stuff, dumbells, kids, neighbours' dogs, bike, what ever :)

Here's a chart to count reps for full rounds.

Minute:
Reps:

1 1
2 3
3 6
4 10
5 15
6 21
7 28
8 36
9 45
10 55
11 66
12 78
13 91
14 105
15 120
16 136
17 153
18 171
19 190
20 210