Can a wife touch husband's penis in Islam?

Yes.

The wife is allowed in Islam to touch and/or hold the husbands penis and enjoy this in any way they like. However, this should be per acceptance and pleasure of both partners; the husband and the wife. It is allowed for partners to perforn oral sex especially as foreplay.

it is to be noted that anal sex (inserting the penis in wife's anus) is forbidden in Islam.

see related question below for more information.