- The silliest thing (that) I have done is kiss my boss.
Note, the noun phrase kiss my boss refers back to the noun phrase silliest thing.
- The silliest thing (that) I have done is kissing my boss.
- Kissing my boss is the silliest thing (that) I have done.
Below, the phrase studying geography sits in a subject position, which makes it a noun. Nouns that end in -ing are called Gerunds. The following is a parallel construction,Note, the noun phrase the silliest thing (that) I have done, which houses the verb phrase 'have done', refers back to gerundival Kissing my boss.
I think (that) studying geography is more interesting than studying math.