Best Way to spend 800k in f2p

What is the best thing i can do with 800k in f2p?

Depends if you like raising your skills or like money more. If you like raising your skills try spending it on something like smithing or mage. If not merch it for more and save up for that nice rare item ;]

Do the air rune thing. Buy at 10, sell at 11. Invest like 500k into this and you'll earn 50k

The sales should go through in a day or 2-the buying part takes the longest.

look on g.e. charts for something to merch with

yea the everlasting quest to make money...here are my top 5 for any1 who is stuck(dont blame them)
1.green dragons(<3 the guide zariff)
2:making air orbz (my personel fav, doesnt stack up against greenies tho :l)mage xp also
3:making nats(great money and a great way to train that annoying rcng skill)
4:merching, ive failed many times at this, but after 2 hrs looking thru ge and with consulting with my advisors(mum and dad 8D) i find itemz
5:slayer, its slayer pl0x the skill cape shows u ghetto stopping a creature thing u can just tell from that its a great way to make cash! hope i helped