Although there are only a few years left before I retire, I still can't wait to retire now.

I think 'now' is unneeded here.

After I retire, I would like to travel around.

Around where? The world? Your country? Would sound a lot better if you were more specific.

First of all, I plan to take a deeplong trip insidearound China by spendingthat will last three to five years.

Sounds more fluent this way

After that, I wouldant to/will travel all over the world.

I would choose thill choose developed, civilized, beautiful countries as my priority destinations, and following this rule, travel to as many countries as I couldan.

You're talking about something you are planning to do becoming a reality, I would not use 'would', instead use 'I want to' or 'I will', because the situation is not so hypothetical. The same for replaced 'could' with 'can'.

When I feel that it'slike I've done enough for traveling, I will go back to my hometown toand spend the rest of my life there.

The first sentence doesn't sound natural.
To spend the rest of my life... You need a destination or action you're spending it on directly related e.g. 'there'
It would also make sense to say 'I will spend the rest of my life in my hometown (once I return)'

If my health so permits, I still want to do somewould like to start a small business, but at that time, I wouldn't be doing the business for myself.

I think it's more of a hypothetical, unsure idea so I would you 'I would like' instead.
I think 'but at that time' is unneeded as well.

We can't go back to the past, as well asnor retain our youth.

Sounds more natural to me

The only thing we can do will beis to do more useful things and leave more good things tofor/in this world.