Here's how the conversation can be completed using "can" or a form of "be able to."

**Harriet:** Hello, David. I'm sorry I haven't (►) **been able to come** (come) and see you before. I've been really busy lately. How are you?
**David:** I'm OK, thanks. (1) **I can walk** (I / walk) around now. The doctor says (2) **I'll be able to go** (I / go) back to work soon. It'll be nice (3) **to get** (get) out again. I hate being stuck here like this. I haven't (4) **been able to do** (do) anything interesting.

### Explanation:
1. **I can walk** - This indicates that David is currently capable of walking.
2. **I'll be able to go** - This is a future ability; David is saying that he will be able to return to work soon.
3. **to get** - This is an infinitive form which does not need "can" or "be able to."
4. **been able to do** - This indicates that up until now, David has been unable to do anything interesting.

"Can" is used for present abilities, while "be able to" is often used for future or past abilities, depending on the context.