As a Singaporean working overseas with no CPF, what should I do with my cash savings when I return? Should I top up CPF?

Why don't topup your CPF now? Topup your special account.
You will earned a guaranteed rate of 4%. For the first $60k in your CPF combined balance (with only up to $20k max from OA), the monies will earn an extra 1%. Since you are still young, u will not be near the limit yet. Thus it is worthwhile to topup to earn effective 5% return. Risk-free, regardless of market conditions.

In additional u will receive tax relief for the topped up amount (not for now since u are earning overseas, but will be useful in future when u return). Eg, your taxable income is $40k, for every dollar u earn above 40k is taxed at 7%. Therefore, if u topped up $1000 into your special account, u get $1000 tax relief, u saved 7% of $1000 = $70 tax money. Effectively have gotten 7% return on your money, in additional u will earn 4%-5% interest yearly guaranteed.

Consider CPF as a baseline retirement planning investment. Consider other investment after maximising the benefits u can get from CPF. Do add me on facebook if u would like to discuss further.

A few quentions. Does your savings include your emergency funds? Do you expect to return to Singapore soon, or will you be overseas for a few more years? Which Country are you based in?

If you are based in developed countries, like europe or the states, You might not want to put all your savings into CPF and want to look into investing from those countries you are in, while you are still a resident. Do check with them what will happen to your investment if you leave the country. (Usually should be able to keep it to sell in future. Only can't do additional trades.)

Suggesting this as it is actually cheaper and and easier to buy stocks when you are in europe or US. If not, Probably just put your extra Savings (minus emergency) in CPF.