The classic column-addition stage: two-digit sums, carrying included, laid out in print-friendly columns with an answer key page.
Two-digit addition is where place value starts doing real work — tens and ones land in their own columns and carrying appears. The problems here are drawn across the full 10–99 range, so regrouping shows up naturally rather than on every line.
Set three or four rows to stretch stronger students: multi-addend columns are the bridge to the running mental sums that abacus students train.