If you want to invest using AlertPay and receive the money in PayPal, certainly, you cannot invest in HYIP/AutoSurf sites, because you will only get paid through the payment processor you buy upgrades/ads package with.

You can invest in PTC sites, but I would suggest you to not buy/rent referrals, because there is high chance that you will get inactive referrals. The only way you can get profits from PTC sites is to buy upgrades and make RCB offers. But the PTC site has to be the ones which pay with PayPal (since you want your money back in PayPal) and you can use your AlertPay to pay your referrals. But I cannot suggest you any good PTC sites which offer high click rates.

The last option is like nick said, to exchange your money to PayPal. But you have to be ready to get less money, because I think AlertPay is not favorable.