It's going to be pretty much impossible to get significant growth without risking the capital in the short to medium term. But premium bonds are not a bad option, and the return is likely to be close to the average payout rate the closer you are to the maximum holding.
But if your pot is a few thousand, chances are that you'll win little or nothing, but the average payout rate is 1.4%, which is higher than any other instant access savings account.
You're effectively gambling the interest you would definitely receive on your savings account, so it's up to the individual how they feel about taking that risk.
Say you have £10k of savings. The best instant access savings account pays 1.2%, so you'll get £120 per year in interest.
You'd have to get 5 x £25 PB prizes in a year to beat this rate, but typically, you'd probably get 2 or 3 prizes per year, so an effective interest rate of 0.5-0.75%, so less, but better than a lot of savings accounts out there. Plus there's a small chance you'll win big, and a reasonable chance you'll beat the best savings account.
I have built up £14k PBs over the last year. In 2019, my prizes equated to an interest rate of 2.1% so better than instant access savings. This year, I've had 2 x £25 prizes so far, so obviously losing out on a bit of interest, but there's still the rest of the year to go.