If you’ve got the 529 covered, then you should consider opening a standard brokerage account in your child’s name at Vanguard. You’ll stay on the account until s/he turns 18. If you want to be able to show the additions to it over time, you can always save down a monthly record of statements and upload them to a Google (Drive) account you start for child now and also handoff to him/her when the time is right. You can also create a Google Sheets file to document the additions over time and make some charts.
As far as an investment split, you should consider leaving it 100% invested in stocks (a normal ETF such as VTI will do just fine). If you have an 18 year investment horizon, you can stomach any market swings, and history has shown that 100% stock allocation has performed best relative to any stock/bond mix.